[WIP] Fix NPE in StatsAppState during app initialization

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>StatsAppState.setEnabled fires NPE if called in app init</issue_title>
<issue_description>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

 
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

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</comments>

• Fixes StatsAppState.setEnabled fires NPE if called in app init #2613

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