XML : DialogFragment: button.setOnClickListener throws NullPointerException

I have an Activity with a button like this:

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:tools="http://schemas.android.com/tools"  android:layout_width="match_parent"  android:layout_height="match_parent"  tools:context=".MainActivity">    <Button      android:id="@+id/button"      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:layout_alignParentBottom="true"      android:layout_centerHorizontal="true"      android:onClick="onButtonClickListener"      android:text="Show Fragment" />  </RelativeLayout>    

And my onButtonClickListener method from xml is(here I create the DialogFragment and set onClickListener for a clear button from dialog):

   public void onButtonClickListener(View view) {      FragmentManager fm = getFragmentManager();      MyDialogFragment dialogFragment = new MyDialogFragment ();      dialogFragment.show(fm, "Sample Fragment");        final EditText messageText = (EditText) findViewById(R.id.editText);      Button clearButton = (Button) findViewById(R.id.button_clear);       clearButton.setOnClickListener(new View.OnClickListener() {          @Override          public void onClick(View v) {              messageText.setText(null);          }      });  }    

My problem is that when DialogFragment shows it throws NullPointerException:

   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference          at com.example.cristi.dialogfragmenttest2.MainActivity.onButtonClickListener(MainActivity.java:51)    

At this line of code: clearButton.setOnClickListener(new View.OnClickListener() {

My xml for DialogFragment is:

  <?xml version="1.0" encoding="utf-8"?>  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:layout_width="match_parent"  android:layout_height="match_parent">    <EditText      android:id="@+id/editText"      android:layout_width="match_parent"      android:layout_height="wrap_content"      android:layout_alignParentTop="true"      android:layout_centerHorizontal="true"      android:maxLength="200" />    <Button      android:id="@+id/button_clear"      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:text="Clear"      android:layout_below="@+id/editText"      android:layout_centerHorizontal="true" />    </RelativeLayout>    

What could be the problem with the setOnClickListener? Why does it says that my button from DialogFragment is null?

Any reference or advice is welcome! Thanks in advance!

Edit:

-MyDialogFragment code:

  public class MyDialogFragment extends DialogFragment {    @Override  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {      View rootView = inflater.inflate(R.layout.dialog_fragment, container, false);      getDialog().setTitle("Enter message");      return rootView;  }