DarkUIPreferenceController null exception

The DarkUIPreferenceController doesnt need to show dialog in all cases
we added a null check for mFragment

Fixes: 144899194
Test: atest DarkUIPreferenceControllerTest
Change-Id: Ia5ed89bd46f28f4b92d955911223f9157e55c622
(cherry picked from commit c49f266af0)

src/com/android/settings/display/DarkUIPreferenceController.java

@@ -100,7 +100,7 @@ public class DarkUIPreferenceController extends TogglePreferenceController imple
 
     private void showDarkModeDialog() {
         final DarkUIInfoDialogFragment frag = new DarkUIInfoDialogFragment();
-        if (mFragment.getFragmentManager() != null) {
+        if (mFragment != null && mFragment.getFragmentManager() != null) {
             frag.show(mFragment.getFragmentManager(), getClass().getName());
         }
     }