Android : Android: Change a Button's text in a Dialog

I have in my Android App a Dialog which contains beside other things a Button. Now I want to change the text of that Button, but I get a NullPointer Exception.

The Button opens up a new Dialog (Datepicker) which gives the MainActivity a date (as a String) which I want then to use for the Button. Obviously I can declare the button and define him, but not interact. Within the try-statement (not sure whether it is a statement) the first line worked, but if I have the second line not commented and call it it crashes (or calls an exception, I added the try part later). I hope this is specific enough.

Here is the code from the button, the MainActivity and the Dialog the button lives in:

Dialog (.java)

public class CreateDialogFragment extends DialogFragment
{
    @Override
    public Dialog onCreateDialog(Bundle savedInstance)
    {
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        LayoutInflater layoutInflater = getActivity().getLayoutInflater();

        builder.setView(layoutInflater.inflate(R.layout.dialog_create, null))
                .setMessage(R.string.create)
                .setPositiveButton(R.string.create, new DialogInterface.OnClickListener()
                {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i)
                    {

                    }
                })
                .setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener()
                {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i)
                    {

                    }
                });

        return builder.create();
    }
}

Dialog (.xml)

<?xml version="1.0" encoding = "utf-8"?>
<RelativeLayout
    xmlns:android = "http://schemas.android.com/apk/res/android"
    xmlns:tools = "http://schemas.android.com/tools"
    android:id = "@+id/dialog_create"
    android:layout_width = "match_parent"
    android:layout_height = "match_parent"
    android:background="@color/create_bg_1"
    tools:context = ".CreateDialogFragment"
    >

    <EditText
        android:id = "@+id/dialog_create_name"
        android:layout_width = "match_parent"
        android:layout_height = "@dimen/textFieldHight"
        android:text = "@string/create_name"
        android:background="@color/white"
        android:layout_marginTop="4dp"
        android:layout_marginBottom="4dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="4dp"
        />


    <Button
        android:layout_width = "wrap_content"
        android:layout_height = "@dimen/textFieldHight"
        android:ems = "6"
        android:text = "@string/date"
        android:background="@color/white"
        android:id = "@+id/create_date_button"
        android:onClick = "pickDate"
        android:layout_below = "@id/dialog_create_name"
        />


</RelativeLayout>

MainActivity -> Method that is causing the problem and method that calls the dialog (.java - partially)

public void openCreate(View view)
{
    createDialogFragment = new CreateDialogFragment();
    createDialogFragment.show(fragmentManager, "TEST");

    updateSelectedButton(3);
}

This causes the problem:

public void dateForWhatever(int year, int month, int day)
{
    DateFormatter dateFormatter = new DateFormatter();
    String date = dateFormatter.toStringDE(year, month, day);

    try
    {
        Button createDateButton = (Button) findViewById(R.id.create_date_button);
        createDateButton.setText(date);
    }
    catch (Exception cannotFindButton)
    {
        System.out.println("Geh kacken\n" + year + " " + month + " " + day);
    }
}

I hope this is specific enough and I did not forget any important code samples.

I appreciate any kind of productive help :)

John