Do not show enqueue button if play queue not ready

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

@@ -70,6 +70,15 @@ public final class PlayerHolder {
         return player != null;
     }
 
+    /**
+     * Use this method to only allow the user to manipulate the play queue (e.g. by enqueueing via
+     * the stream long press menu) when there actually is a play queue to manipulate.
+     * @return true only if the player is open and its play queue is ready (i.e. it is not null)
+     */
+    public boolean isPlayQueueReady() {
+        return player != null && player.getPlayQueue() != null;
+    }
+
     public boolean isBound() {
         return bound;
     }