#2562 Reset thread warden on shutdown

jme3-core/src/main/java/com/jme3/app/SimpleApplication.java

@@ -244,6 +244,12 @@ public abstract class SimpleApplication extends LegacyApplication {
         simpleInitApp();
     }
 
+    @Override
+    public void stop(boolean waitFor) {
+        SceneGraphThreadWarden.reset();
+        super.stop(waitFor);
+    }
+
     @Override
     public void update() {
         if (prof != null) {