android - getDialog does not return an instance of AlertDialog

I've been working on google's course sunshine app and wanted to put my personal touch in it so i made the user specify his city by using a hybrid of EditTextPreference and AutoCompleteTextView shown in here:

public class AutoCompleteEditTextPreference extends EditTextPreference {

private static String[] list;
private boolean isValid = true;
private Dialog dialog;

public AutoCompleteEditTextPreference(Context context) {
    super(context);
}

public AutoCompleteEditTextPreference(Context context, AttributeSet attrs) {
    super(context, attrs);
}

public AutoCompleteEditTextPreference(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}

/**
 * the default EditTextPreference does not make it easy to
 * use an AutoCompleteEditTextPreference field. By overriding this method
 * we perform surgery on it to use the type of edit field that
 * we want.
 */
protected void onBindDialogView(View view) {
    super.onBindDialogView(view);

    // find the current EditText object
    final EditText editText = (EditText) view.findViewById(android.R.id.edit);
    // copy its layout params
    ViewGroup.LayoutParams params = editText.getLayoutParams();
    ViewGroup vg = (ViewGroup) editText.getParent();
    String curVal = editText.getText().toString();
    // remove it from the existing layout hierarchy
    vg.removeView(editText);

    // construct a new editable autocomplete object with the appropriate params
    // and id that the TextEditPreference is expecting
    mACTV = new AutoCompleteTextView(getContext());
    mACTV.setLayoutParams(params);
    mACTV.setId(android.R.id.edit);
    mACTV.setText(curVal);


    Arrays.sort(list);

    isValid  = isValid(mACTV.getText().toString());


    mACTV.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {

        }

        @Override
        public void afterTextChanged(Editable s) {
            isValid = isValid(s.toString());
            validate();
        }
    });

    mACTV.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            isValid = isValid(mACTV.getText().toString());
            validate();
        }
    });

    ArrayAdapter<String> adapter = new ArrayAdapter<>(getContext(),
            android.R.layout.simple_dropdown_item_1line, list);
    mACTV.setAdapter(adapter);

    // add the new view to the layout
    vg.addView(mACTV);
}

private boolean isValid(CharSequence text) {
    return !text.equals("") && Arrays.binarySearch(list, text.toString()) > 0;
}

@Override
protected void showDialog(Bundle state) {
    super.showDialog(state);
    validate();
}

private void validate() {
    dialog = getDialog();
    Toast.makeText(getContext(), Boolean.toString(dialog instanceof AlertDialog), Toast.LENGTH_SHORT).show();

    if (dialog instanceof AlertDialog) {
        Button btn = ((AlertDialog) dialog).getButton(AlertDialog.BUTTON_POSITIVE);
        btn.setEnabled(isValid);
    }
}

/**
 * Because the baseclass does not handle this correctly
 * we need to query our injected AutoCompleteTextView for
 * the value to save
 */
protected void onDialogClosed(boolean positiveResult) {
    super.onDialogClosed(positiveResult);

    if (positiveResult && mACTV != null) {

        String value = mACTV.getText().toString();

        if (callChangeListener(value))
            setText(value);
    }
}


static void prepareCountriesList(Context context) {

    List<String> lines = new ArrayList<>();

    try {

        InputStream inputStream = context.getAssets().open("cities.txt");
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

        String line;

        while ((line = bufferedReader.readLine()) != null) {
            lines.add(line);
        }

    } catch (IOException e) {
        e.printStackTrace();
    }

    list = lines.toArray(new String[lines.size()]);
}

/**
 * again we need to override methods from the base class
 */
public EditText getEditText() {
    return mACTV;
}

private AutoCompleteTextView mACTV = null;
private final String TAG = "AutoCompleteEditTextPreference";
}

so everything was going great until the last part where i wanted to disable the ok button

private void validate() {
    dialog = getDialog();
    Toast.makeText(getContext(), Boolean.toString(dialog instanceof AlertDialog), Toast.LENGTH_SHORT).show();

    if (dialog instanceof AlertDialog) {
        Button btn = ((AlertDialog) dialog).getButton(AlertDialog.BUTTON_POSITIVE);
        btn.setEnabled(isValid);
    }
}

so i try the method getDialog();

and it returns a dialog that is not null and not an instance of AlertDialog

anyhelp please on getting the dialog properly or another way to disable the ok button programmatically

It's ok found the problem; it was that i used

import android.support.v7.app.AlertDialog;

instead of

import android.app.AlertDialog;

thanks for anyone who tried to help