NSOpenPanel model window kills main application window

Question

I'm new to Mac application programming. I'm using NSOpenPanel to select a file from system. Once i select a file, and press Cancel/Open button, completionHandler block is getting executed.

When OpenPanel window is closed, i am facing following issues:

1. my main application window is also getting closed even though result of mainAppWindow.visible is 1
2. Also, I am not receiving the window close delegate method for main App window.

Please find below the code snippet which I have used to create NSOpenPanel.

-(void)openFile
{
    //this gives you a copy of an open file dialogue
    NSOpenPanel*openPanel = [NSOpenPanel openPanel];

    //set the title of the dialogue window
    openPanel.title = @"Choose a .zip formate file";

    //shoud the user be able to resize the window?
    openPanel.showsResizeIndicator = NO;

    //should the user see hidden files (for user apps - usually no)
    openPanel.showsHiddenFiles = NO;

    //can the user select a directory?
    openPanel.canChooseDirectories = NO;

    //can the user create directories while using the dialogue?
    openPanel.canCreateDirectories = NO;

    //should the user be able to select multiple files?
    openPanel.allowsMultipleSelection = NO;

    //an array of file extensions to filter the file list
    openPanel.allowedFileTypes = @[@"zip"];

    [openPanel beginWithCompletionHandler:^(NSInteger result) {

        //if the result is NSOKButton
        //the user selected a file

            if (result==NSModalResponseOK) {

                //Do something
            }
            else if (result== NSModalResponseCancel)
            {
                //do something
            }
    }];  
}