AlertDialog.Builder with custom layout and EditText; cannot access view

editText is a part of alertDialog layout so Just access editText with reference of alertDialog

EditText editText = (EditText) alertDialog.findViewById(R.id.label_field);

Update:

Because in code line dialogBuilder.setView(inflater.inflate(R.layout.alert_label_editor, null));

inflater is Null.

update your code like below, and try to understand the each code line

AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(this);
// ...Irrelevant code for customizing the buttons and title
LayoutInflater inflater = this.getLayoutInflater();
View dialogView = inflater.inflate(R.layout.alert_label_editor, null);
dialogBuilder.setView(dialogView);

EditText editText = (EditText) dialogView.findViewById(R.id.label_field);
editText.setText("test label");
AlertDialog alertDialog = dialogBuilder.create();
alertDialog.show();

Update 2:

As you are using View object created by Inflater to update UI components else you can directly use setView(int layourResId) method of AlertDialog.Builder class, which is available from API 21 and onwards.

You can write:

AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(this);

 // ...Irrelevant code for customizing the buttons and title

LayoutInflater inflater = this.getLayoutInflater(); 

View dialogView= inflater.inflate(R.layout.alert_label_editor, null);                    
dialogBuilder.setView(dialogView);

Button button = (Button)dialogView.findViewById(R.id.btnName);

   button.setOnClickListener(new View.OnClickListener() {
       @Override
       public void onClick(View view) {

         //Commond here......

       }
   });

EditText editText = (EditText)
dialogView.findViewById(R.id.label_field); 

editText.setText("test label"); 

dialogBuilder.create().show();

Use this one

   AlertDialog.Builder builder = new AlertDialog.Builder(activity);
    // Get the layout inflater
    LayoutInflater inflater = (activity).getLayoutInflater();
    // Inflate and set the layout for the dialog
    // Pass null as the parent view because its going in the
    // dialog layout
    builder.setTitle(title);
    builder.setCancelable(false);
    builder.setIcon(R.drawable.galleryalart);
    builder.setView(inflater.inflate(R.layout.dialogue, null))
    // Add action buttons
            .setPositiveButton("OK", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int id) {

                    }
                }
            });
    builder.create();
    builder.show();

In case any one wants it in Kotlin :

val dialogBuilder = AlertDialog.Builder(this)
// ...Irrelevant code for customizing the buttons and title
val dialogView = layoutInflater.inflate(R.layout.alert_label_editor, null)
dialogBuilder.setView(dialogView)

val editText =  dialogView.findViewById(R.id.label_field)
editText.setText("test label")
val alertDialog = dialogBuilder.create()
alertDialog.show()

Reposted @user370305's answer.