fix npe in sliding overlay (#899)

android/app/src/main/java/com/reactnativenavigation/views/slidingOverlay/SlidingOverlaysQueue.java

             public void run() {
                 SlidingOverlay currentOverlay = queue.peek();
 
+                if(currentOverlay == null) {
+                    return;
+                }
+                
                 if (currentOverlay.isShowing()) {
                     pendingHide = true;
                 }