DialogFragment PositiveButton NullPointerException

Question

我在PositiveButton.setBackgroundColor上遇到了一个用户（三星，Android 5）的崩溃。它只发生过一次，我不能重复它。 这是代码：

public Dialog onCreateDialog(Bundle savedInstanceState) {

...
        activity = getActivity();
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(activity);
        alertDialogBuilder.setCustomTitle(layout);
        alertDialogBuilder.setView(returnDialogView());
        alertDialogBuilder.setPositiveButton(R.string.btn_save, null);
        mAlertDialog = alertDialogBuilder.create();
        mAlertDialog.setOnShowListener(new DialogInterface.OnShowListener() {

            @Override
            public void onShow(DialogInterface d) {

                mPositiveButton = mAlertDialog.getButton(AlertDialog.BUTTON_POSITIVE);
                mPositiveButton.setBackgroundColor(getResources().getColor(R.color.brand_gray));
                mPositiveButton.setOnClickListener(new View.OnClickListener() {

                    @Override
                    public void onClick(View view) {
                        if(customerEditMode){
                            showFieldsOverCustomer();
                        }else {
                            save();
                        }
                    }
                });
private final TextWatcher  mTextEditorWatcher = new TextWatcher() {
...
        public void afterTextChanged(Editable s)
        {
            if (mInputCardNumber.getText().toString().equals("")){
                mPositiveButton.setBackgroundColor(getResources().getColor(R.color.brand_gray)); //Here is the NullPointer Exception
private View returnDialogView() {
...
mInputCardNumber.addTextChangedListener(mTextEditorWatcher);
}

代码工作正常，没有人遇到问题。这是例外：

Caused by java.lang.NullPointerException    
my.package.ui.fragment.dialog.NewTransactionDialogFragment$2.afterTextChanged (NewTransactionDialogFragment.java:275)
    android.widget.TextView.sendAfterTextChanged (TextView.java:8699)
    android.widget.TextView.setText (TextView.java:4701)
    android.widget.TextView.setText (TextView.java:4545)
    android.widget.EditText.setText (EditText.java:101)
    android.widget.TextView.setText (TextView.java:4520)
    ...

在解除对话框之前，该按钮永远不会出现。我不知道它为什么会发生。我该如何解决呢。