Add Nullable annotation for MainFragment.getMainActivity and create MainFragment.requireMainActivity that is NonNull

app/src/main/java/com/amaze/filemanager/ui/fragments/MainFragment.java

@@ -1759,10 +1759,17 @@ public class MainFragment extends Fragment
     super.onDetach();
   }
 
+  @Nullable
   public MainActivity getMainActivity() {
     return (MainActivity) getActivity();
   }
 
+  @SuppressWarnings("unused")
+  @NonNull
+  public MainActivity requireMainActivity() {
+    return (MainActivity) requireActivity();
+  }
+
   @Nullable
   public ArrayList<LayoutElementParcelable> getElementsList() {
     return mainFragmentViewModel.getListElements();