nullPointerException on calling getActivity().runOnUiThread{new Runnable inside Fragment

i'm trying to update my ListPreference summary inside my PreferenceFragment with timer while i'm inside my fragment everything is fine but as soon as i press the back button my app crashes and it throws a null pointer exception saying getActivity() retured null. i also tried suppressing it by setting the timer only when getActivity() won't return null but it did'nt work.

here's my code:

public class ButtonsFragment extends PreferenceFragmentCompat {

@Override
public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
    setPreferencesFromResource(R.xml.misc_option_settings, rootKey);
    
    if (getActivity() == null)
        return;
    
    new Timer().scheduleAtFixedRate(new TimerTask() {
        @Override
        public void run() {
            getActivity().runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    ListPreference rows = findPreference("btn_rows");
                    rows.setSummary("current button rows: " + rows.getValue());

                    ListPreference color = findPreference("btn_colors");
                    color.setSummary("current button color: " + color.getEntry());
                }
            });
        }
    }, 0, 100);
}

}

would appreciate if you could help.

You should check for activity not to be null inside run method. check for activity not to be null before calling getActivity().runOnUiThread.