Fix NPE and add some @Nullables

Fix NullPointerException in PlayerHolder.getQueueSize() and add `@Nullable` here and there so that the linter reports risks of NPEs

Author: Stypox

app/src/main/java/org/schabi/newpipe/player/Player.java

@@ -260,7 +260,8 @@ public final class Player implements
     // Playback
     //////////////////////////////////////////////////////////////////////////*/
 
-    private PlayQueue playQueue;
+    // play queue might be null e.g. while player is starting
+    @Nullable private PlayQueue playQueue;
     private PlayQueueAdapter playQueueAdapter;
     private StreamSegmentAdapter segmentAdapter;
 
@@ -4202,6 +4203,7 @@ public boolean popupPlayerSelected() {
     }
 
 
+    @Nullable
     public PlayQueue getPlayQueue() {
         return playQueue;
     }

app/src/main/java/org/schabi/newpipe/player/helper/PlayerHolder.java

@@ -38,12 +38,12 @@ public static synchronized PlayerHolder getInstance() {
     private static final boolean DEBUG = MainActivity.DEBUG;
     private static final String TAG = PlayerHolder.class.getSimpleName();
 
-    private PlayerServiceExtendedEventListener listener;
+    @Nullable private PlayerServiceExtendedEventListener listener;
 
     private final PlayerServiceConnection serviceConnection = new PlayerServiceConnection();
     private boolean bound;
-    private MainPlayer playerService;
-    private Player player;
+    @Nullable private MainPlayer playerService;
+    @Nullable private Player player;
 
     /**
      * Returns the current {@link MainPlayer.PlayerType} of the {@link MainPlayer} service,
@@ -75,7 +75,11 @@ public boolean isBound() {
     }
 
     public int getQueueSize() {
-        return isPlayerOpen() ? player.getPlayQueue().size() : 0;
+        if (player == null || player.getPlayQueue() == null) {
+            // player play queue might be null e.g. while player is starting
+            return 0;
+        }
+        return player.getPlayQueue().size();
     }
 
     public void setListener(@Nullable final PlayerServiceExtendedEventListener newListener) {