Fix NPE in MediaSessionPlayerUi while destroying player

app/src/main/java/org/schabi/newpipe/player/mediasession/MediaSessionPlayerUi.java

@@ -38,7 +38,9 @@ public class MediaSessionPlayerUi extends PlayerUi
         implements SharedPreferences.OnSharedPreferenceChangeListener {
     private static final String TAG = "MediaSessUi";
 
+    @Nullable
     private MediaSessionCompat mediaSession;
+    @Nullable
     private MediaSessionConnector sessionConnector;
 
     private final String ignoreHardwareMediaButtonsKey;
@@ -198,6 +200,11 @@ public class MediaSessionPlayerUi extends PlayerUi
             return;
         }
 
+        if (sessionConnector == null) {
+            // sessionConnector will be null after destroyPlayer is called
+            return;
+        }
+
         // only use the fourth and fifth actions (the settings page also shows only the last 2 on
         // Android 13+)
         final List<NotificationActionData> newNotificationActions = IntStream.of(3, 4)