Android - Kotlin: Spinner keeps giving me null exception even do my Adapter is not empty

Question

I know this question is asked many times but I didn't find why is happening null exception. spinnerCourses is the ID of the Spinner and when I try to set the adapter I get null exception.

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    setSupportActionBar(toolbar)

    val dm = DataManager()
    val adapterCourses = ArrayAdapter<CourseInfo>(this, android.R.layout.simple_spinner_item,
        dm.courses.values.toList())
    adapterCourses.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)

    spinnerCourses.adapter = adapterCourses
}

override fun onCreateOptionsMenu(menu: Menu): Boolean {
    // Inflate the menu; this adds items to the action bar if it is present.
    menuInflater.inflate(R.menu.menu_main, menu)
    return true
}

override fun onOptionsItemSelected(item: MenuItem): Boolean {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    return when (item.itemId) {
        R.id.action_settings -> true
        else -> super.onOptionsItemSelected(item)
    }
}

}

And here is the error log::::::::::::::::::::::::::::::::::::::::::::::::::::::

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.notekeeper/com.example.notekeeper.MainActivity}: java.lang.IllegalStateException: spinnerCourses must not be null
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
    at android.app.ActivityThread.-wrap11(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
 Caused by: java.lang.IllegalStateException: spinnerCourses must not be null
    at com.example.notekeeper.MainActivity.onCreate(MainActivity.kt:26)
    at android.app.Activity.performCreate(Activity.java:6975)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
    at android.app.ActivityThread.-wrap11(Unknown Source:0) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:164) 
    at android.app.ActivityThread.main(ActivityThread.java:6541) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

Layout code::::::::::::::::::::::::::::::::::::::::::::::::

    <?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FirstFragment">

    <EditText
        android:id="@+id/textNoteText"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:gravity="start|top"
        android:hint="Note text"
        android:inputType="textMultiLine"
        app:layout_constraintEnd_toEndOf="@+id/spinnerCourses"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="@+id/spinnerCourses"
        app:layout_constraintTop_toBottomOf="@+id/textNoteTitle" />

    <Spinner
        android:id="@+id/spinnerCourses"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="32dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="32dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/textNoteTitle"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:gravity="start|top"
        android:hint="Note title"
        android:inputType="textMultiLine"
        app:layout_constraintEnd_toEndOf="@+id/spinnerCourses"
        app:layout_constraintStart_toStartOf="@+id/spinnerCourses"
        app:layout_constraintTop_toBottomOf="@+id/spinnerCourses" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

Your spinner is in Fragment, Not in your Main activity. Thats why you getting Null pointer exception. Please wrote your below code to FirstFragment or use activity_main.xml for your spinner and other widget.

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

    val dm = DataManager()
    val adapterCourses = ArrayAdapter<CourseInfo>(this, android.R.layout.simple_spinner_item,
    dm.courses.values.toList())
    adapterCourses.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)
    spinnerCourses.adapter = adapterCourses

    }