8255248: NullPointerException in JFXPanel due to race condition in HostContainer

8334593: Adding, removing and then adding a JFXPanel again leads to NullPointerException

Reviewed-by: angorya, kcr

Author: prsadhuk

modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/GlassScene.java

@@ -249,8 +249,10 @@ final SceneState getSceneState() {
     }
 
     final void updateSceneState() {
-        // should only be called on the event thread
-        sceneState.update();
+        // should only be called on the FX application thread
+        if (sceneState != null) {
+            sceneState.update();
+        }
     }
 
     protected View getPlatformView() {