glTFs with an AnimComposer but no skeleton throw an exception

[stephengold]

Reported by @capdevon at the JME forum:

https://hub.jmonkeyengine.org/t/assimp-based-model-import-and-the-monkeywrench-library/46930/66

A copy of the stack trace:

SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
com.jme3.asset.AssetLoadException: An exception has occurred while loading asset: Models/Zombiegirl/Running.gltf
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:274)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:388)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:439)
	at com.capdevon.test.Test_AssetManager.simpleInitApp(Test_AssetManager.java:69)
	at com.jme3.app.SimpleApplication.initialize(SimpleApplication.java:240)
	at com.jme3.system.lwjgl.LwjglWindow.initInThread(LwjglWindow.java:607)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:710)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.io.IOException: Assimp failed to import an asset from "Models/Zombiegirl/Running.gltf":
 No materials remaining
	at com.github.stephengold.wrench.LwjglAssetLoader.loadScene(LwjglAssetLoader.java:144)
	at com.github.stephengold.wrench.LwjglAssetLoader.load(LwjglAssetLoader.java:95)
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:272)
	... 7 more

[stephengold]

This appears to be a direct result of assimp/assimp#5438 .
A fix has been submitted: assimp/assimp#5439 .
I'll leave this issue open until a fix makes its way into a stable build of LWJGL---I estimate 6-to-12 months.

In the mean time, the best workaround is to disable RemoveRedundantMaterials post processing:

    int ppFlags = Assimp.aiProcess_CalcTangentSpace
            | Assimp.aiProcess_JoinIdenticalVertices
            | Assimp.aiProcess_Triangulate
            | Assimp.aiProcess_ValidateDataStructure
            | Assimp.aiProcess_SortByPType;
    LwjglAssetKey key = new LwjglAssetKey("Models/Zombiegirl/Running.gltf", ppFlags);
    Spatial m = assetManager.loadModel(key);

[stephengold]

On 1 February, Kim wrote and integrated his own solution (PR 5451) for the issue. 5451 introduced a new bug (issue 5461), for which I submitted PR 5467. 5467 has now been integrated into assimp's master branch.

[stephengold]

The fix made it into LWJGL v3.3.4.
Today I verified this using a custom build of jMonkeyEngine.
The next step is to update the LWJGL version used in jMonkeyEngine: jMonkeyEngine/jmonkeyengine#2298