Fix for gltf empty scene

[tlf30]

This is a fix for when the gltf exporter creates an empty scene node from blender. For example:

    "scenes" : [
        {
            "name" : "KB3D_NeoShanghai-Native",
            "nodes" : [
                5
            ]
        },
        {
            "name" : "Scene"
        }
    ],

This will make the exporter skip attempting to read the nodes from the empty scene, as there are no nodes in the scene.

[pspeed42]

What happens without this code?

[tlf30]

What happens without this code?

The following error will cause the application to crash:

com.jme3.asset.AssetLoadException: An error occurred loading Models/KB3D_NeoShanghai-Native_BldgSM_H.gltf
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:182)
	at com.jme3.scene.plugins.gltf.GltfLoader.load(GltfLoader.java:110)
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:260)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:374)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:417)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:421)
	at io.tlf.jme.test.Main.simpleInitApp(Main.java:36)
	at com.jme3.app.SimpleApplication.initialize(SimpleApplication.java:239)
	at com.jme3.system.lwjgl.LwjglWindow.initInThread(LwjglWindow.java:520)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:632)
	at com.jme3.system.lwjgl.LwjglWindow.create(LwjglWindow.java:473)
	at com.jme3.app.LegacyApplication.start(LegacyApplication.java:463)
	at com.jme3.app.LegacyApplication.start(LegacyApplication.java:424)
	at com.jme3.app.SimpleApplication.start(SimpleApplication.java:127)
	at io.tlf.jme.test.Main.main(Main.java:145)
Caused by: java.lang.NullPointerException
	at com.jme3.scene.plugins.gltf.GltfLoader.readScenes(GltfLoader.java:214)
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:159)
	... 14 more

Caused by: java.lang.NullPointerException

[stephengold]

1. Please add a test case for the issue.
2. The comment is grammatically incorrect. I suggest:

// If the scene is empty, ignore it.

[tlf30]

@stephengold how does this test look?
I ran it with the added code for the fix removed, and it failed. Added the fix back in and it passed.
Not entirely sure how you wanted the test, as there are inconstancies between the different jme subprojects. I just added this one along side the existing gltf test. If it needs to be elsewhere, or structured differently, please let me know.

[stephengold]

That looks great. Thank you for your contribution to the project!