Added comments and improved the code

app/src/main/java/org/schabi/newpipe/MainActivity.java

@@ -460,7 +460,9 @@ public class MainActivity extends AppCompatActivity {
         if (!isChangingConfigurations()) {
             StateSaver.clearStateFiles();
         }
-        unregisterReceiver(broadcastReceiver);
+        if (broadcastReceiver != null) {
+            unregisterReceiver(broadcastReceiver);
+        }
     }
 
     @Override
@@ -811,8 +813,20 @@ public class MainActivity extends AppCompatActivity {
                     final Fragment fragmentPlayer = getSupportFragmentManager()
                             .findFragmentById(R.id.fragment_player_holder);
                     if (fragmentPlayer == null) {
+                        /*
+                         * We still don't have a fragment attached to the activity.
+                         * It can happen when a user started popup or background players
+                         * without opening a stream inside the fragment.
+                         * Adding it in a collapsed state (only mini player will be visible)
+                         * */
                         NavigationHelper.showMiniPlayer(getSupportFragmentManager());
                     }
+                    /*
+                    * At this point the player is added 100%, we can unregister.
+                    * Other actions are useless since the fragment will not be removed after that
+                     * */
+                    unregisterReceiver(broadcastReceiver);
+                    broadcastReceiver = null;
                 }
             }
         };

app/src/main/java/org/schabi/newpipe/fragments/detail/VideoDetailFragment.java

@@ -304,7 +304,7 @@ public class VideoDetailFragment
         return instance;
     }
 
-    public static VideoDetailFragment getInstanceCollapsed() {
+    public static VideoDetailFragment getInstanceInCollapsedState() {
         final VideoDetailFragment instance = new VideoDetailFragment();
         instance.bottomSheetState = BottomSheetBehavior.STATE_COLLAPSED;
         return instance;

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -392,7 +392,7 @@ public final class NavigationHelper {
     }
 
     public static void showMiniPlayer(final FragmentManager fragmentManager) {
-        final VideoDetailFragment instance = VideoDetailFragment.getInstanceCollapsed();
+        final VideoDetailFragment instance = VideoDetailFragment.getInstanceInCollapsedState();
         defaultTransaction(fragmentManager)
                 .replace(R.id.fragment_player_holder, instance)
                 .runOnCommit(() -> sendPlayerStartedEvent(instance.requireActivity()))