StatsAppState.setEnabled fires NPE if called in app init

Calling    ` stateManager.getState(StatsAppState.class).setEnabled(false);` in simpleInitApp causes this NPE

```
java.lang.NullPointerException: Cannot invoke "com.jme3.scene.Geometry.setCullHint(com.jme3.scene.Spatial$CullHint)" because "this.darkenFps" is null
	at com.jme3.app.StatsAppState.setEnabled(StatsAppState.java:228)
StatsAppState.java:228

 
```