Context mContext = getApplicationContext();
    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(LAYOUT_INFLATER_SERVICE);
    View layout = inflater.inflate(R.layout.dialog_register_app, (ViewGroup) findViewById(R.id.layout_root));

    //Start building dialog
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle(getString(R.string.dialog_register_app_title));
    builder.setView(layout);
    builder.setCancelable(false);
    builder.setPositiveButton(getString(R.string.dialog_register_confirm_button_text), new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int id) {
            //Auslesen der EditText
            EditText oUsername = (EditText) findViewById(R.id.edit_username);
            String strUsername = oUsername.getText().toString(); // This is line where exception is calling
            //Do something with strUsername
        }
   });
   AlertDialog alert = builder.create();
   alert.show();
...
...
...

EditText oUsername = (EditText) layout.findViewById(R.id.edit_username);
            String strUsername = oUsername.getText().toString(); // This is line where exception is calling
...

EditText oUsername = (EditText) layout.findViewById(R.id.edit_username);
String strUsername = oUsername.getText().toString();