android.database.sqlite.SQLiteDatabase.insert on a null object reference

Question

Register

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.content.ContentValues;
import android.content.Intent;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.VideoView;

public class Register extends AppCompatActivity {
    EditText username;
    EditText email;
    EditText password;
    EditText repass;
    EditText role;
    Button register;
    VideoView videologin2;

    TextView change2;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);
        username=(EditText) findViewById(R.id.edtUsername);
        email=(EditText)findViewById(R.id.edtGmail);
        password=(EditText) findViewById(R.id.edtPassword);
        repass=(EditText) findViewById(R.id.edtRepass);
        role=(EditText)findViewById(R.id.edtRole) ;
        register=(Button) findViewById(R.id.btnRegister);
        change2=(TextView)findViewById(R.id.Change2);
        videologin2=(VideoView) findViewById(R.id.videologin2);

        role.setText("user");
        role.setEnabled(false);

        String path="android.resource://com.example.myapplication/"+R.raw.video2;
        Uri uri=Uri.parse(path);
        videologin2.setVideoURI(uri);
        videologin2.start();

        videologin2.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                mp.setLooping(false);
            }
        });
        register.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(username.getText().toString().equals("") || password.getText().toString().equals("")
                        && email.getText().toString().equals("")){
                    Toast.makeText(Register.this, "Vui lòng nhập thông tin còn thiếu", Toast.LENGTH_LONG).show();
                }
                if (username.getText().toString().length() < 5) {
                    Toast.makeText(Register.this, "Tên tài khoản phải dài hơn 5 kí tự", Toast.LENGTH_LONG).show();
                }
                else if (password.getText().toString().length() < 8) {
                    Toast.makeText(Register.this, "Mật khẩu phải dài hơn 8 kí tự", Toast.LENGTH_LONG).show();
                }
                else if (!(email.getText().toString().contains("@") && email.getText().toString().contains("gmail.com"))) {
                    Toast.makeText(Register.this, "Hãy nhập đúng định dạng gmail", Toast.LENGTH_LONG).show();
                }
                else if (!(password.getText().toString().equals(repass.getText().toString()))) {
                    Toast.makeText(Register.this, "Mật khẩu không khớp", Toast.LENGTH_LONG).show();
                }
                else {
                    ContentValues values = new ContentValues();
                    values.put("Username", username.getText().toString());
                    values.put("Password", password.getText().toString());
                    values.put("gmail", email.getText().toString());
                    values.put("role", role.getText().toString());
                    long kq = Login.database.insert("Users",null, values);
                        Intent b = new Intent(Register.this, Login.class);
                        Toast.makeText(Register.this, "Đăng kí thành công", Toast.LENGTH_LONG).show();
                        startActivity(b);

                }
            }
        });
        change2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent b=new Intent(Register.this,Login.class);
                startActivity(b);
            }
        });
    }
    }

Login

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.VideoView;

import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;

public class Login extends AppCompatActivity {
    EditText username;
    EditText password;
    Button login;
    VideoView videologin;
    public String DATABASE_NAME="Mydatabase";
    public String DB_SUFFIX_PATH="/databases/";
    public static SQLiteDatabase database= null;

    TextView change;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        username=(EditText) findViewById(R.id.edtUsername);
        password=(EditText) findViewById(R.id.edtPassword);
        login=(Button) findViewById(R.id.login);
        change=(TextView)findViewById(R.id.Change);
        videologin=(VideoView) findViewById(R.id.videologin);
        proccessCopy();

        String path="android.resource://com.example.myapplication/"+R.raw.video;
        Uri uri=Uri.parse(path);
        videologin.setVideoURI(uri);
        videologin.start();

        videologin.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                mp.setLooping(false);
            }
        });

        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v){
                String etname = username.getText().toString();
                database=openOrCreateDatabase(DATABASE_NAME,MODE_PRIVATE,null);
                Cursor cursor=database.rawQuery("select * from Users where Username='"+etname+"'",null);
                while(cursor.moveToNext()) {
                    Integer ma=cursor.getInt(0);
                    String name=cursor.getString(1);
                    String pass=cursor.getString(2);
                    String role=cursor.getString(4);
                    if((username.getText().toString().equals(name))&&
                            (password.getText().toString().equals(pass))){
                        if(role.equals("admin")){
                            Intent c=new Intent(Login.this,Admin.class);
                            startActivity(c);
                        }
                        else{
                            Intent d=new Intent(Login.this, userPage.class);
                            startActivity(d);
                        }
                    }
                    else if(username.getText().toString().equals(name)
                            &&!password.getText().toString().equals(pass)){
                        Toast.makeText(Login.this,"Sai mật khẩu",Toast.LENGTH_LONG).show();
                    }
                    else if(username.getText().toString().equals("")&&password.getText().toString().equals("")){
                        Toast.makeText(Login.this,"Vui lòng nhập thông tin",Toast.LENGTH_LONG).show();
                    }
                    else if(cursor.getCount()==0){
                        Toast.makeText(Login.this,"Tên tài khoản không tồn tại",Toast.LENGTH_LONG).show();
                    }
                }
            }
        });
        change.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent a=new Intent(Login.this,Register.class);
                startActivity(a);
            }
        });
        }

    public String getDatabasePath(){
        return getApplicationInfo().dataDir+DB_SUFFIX_PATH+DATABASE_NAME;
    }
    private void proccessCopy(){
        try {
            File file=getDatabasePath(DATABASE_NAME);
            if(!file.exists()){
                copyDatabaseFromAssets();
                Toast.makeText(this, "Copy successful", Toast.LENGTH_SHORT).show();
            }
        }
        catch (Exception ex){
            Toast.makeText(this, "Copy Database Fail", Toast.LENGTH_SHORT).show();
        }
    }

    private void copyDatabaseFromAssets() {
        try {
            InputStream inputFile=getAssets().open(DATABASE_NAME);
            String outputFileName=getDatabasePath();
            File file= new File(getApplicationInfo().dataDir+DB_SUFFIX_PATH);
            if(!file.exists())
                file.mkdir();
            OutputStream outFile= new FileOutputStream(outputFileName);
            byte []buffer= new byte[1024];
            int length;
            while ((length=inputFile.read(buffer))>0)
                outFile.write(buffer,0,length);
            outFile.flush();
            outFile.close();
            inputFile.close();

        }catch (Exception e){

        }
    }
}

User class

package com.example.myapplication;

import java.io.Serializable;

public class User implements Serializable {
    private Integer ID_User;
    private String Username;
    private String Password;
    private String gmail;
    private String role;

    public User() {
    }

    public User(Integer ID_User, String username, String password, String gmail, String role) {
        this.ID_User = ID_User;
        Username = username;
        Password = password;
        this.gmail = gmail;
        this.role = role;
    }

    public Integer getID_User() {
        return ID_User;
    }

    public void setID_User(Integer ID_User) {
        this.ID_User = ID_User;
    }

    public String getUsername() {
        return Username;
    }

    public void setUsername(String username) {
        Username = username;
    }

    public String getPassword() {
        return Password;
    }

    public void setPassword(String password) {
        Password = password;
    }

    public String getGmail() {
        return gmail;
    }

    public void setGmail(String gmail) {
        this.gmail = gmail;
    }

    public String getRole() {
        return role;
    }

    public void setRole(String role) {
        this.role = role;
    }

}

my database

i tried to insert the values to my database but it always appear the

 FATAL EXCEPTION: main
                                                                                                    Process: com.example.myapplication, PID: 10120
                                                                                                    java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
                                                                                                        at com.example.myapplication.Register$2.onClick(Register.java:82)
                                                                                                        at android.view.View.performClick(View.java:7659)
                                                                                                        at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1211)
                                                                                                        at android.view.View.performClickInternal(View.java:7636)
                                                                                                        at android.view.View.-$$Nest$mperformClickInternal(Unknown Source:0)
                                                                                                        at android.view.View$PerformClick.run(View.java:30156)
                                                                                                        at android.os.Handler.handleCallback(Handler.java:958)
                                                                                                        at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                                        at android.os.Looper.loopOnce(Looper.java:205)
                                                                                                        at android.os.Looper.loop(Looper.java:294)
                                                                                                        at android.app.ActivityThread.main(ActivityThread.java:8177)
                                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                                        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
                                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)

could anyone tell me the problem of my code? i'm very apreciate