avoid an NPE in GltfLoader.findChildren()

[Xenokieser]

null pointer exception if the GLTF file references a node that doesn't exist.
I got this problem while tring to convert my GLTF file, i got 440 animations on the file with 3 animation that have joint index missmatch.
Maybe it's better to log the errors detail so the user have all the information to correct the error

[tlf30]

In this instance I would think it would be better to throw an IOException as it seems to me the gltf file is corrupt. I don't know what you used to convert your models to gltf, but it is not normal for nodes to be missing. I think even if the model loads with missing nodes in your case, it would just add the possibility for a corrupt gltf to get further in the load process before breaking, which would be even more difficult to troubleshoot.

[Xenokieser]

Its true that if there is any corruption in the file it need to be corrected. But for an end-user of the plug-in, he need to understand what is happening while trying to convert a file. A bulk null-pointer exception in console log is very hard to debug. A more friendly parsing log is better.
As for the throw exception proposal the calling methods don't have any proper exception error handling structure.
If i have more time, i am going to refactor the entire class and add some unit test.

[stephengold]

No refactoring or units tests have been forthcoming. I'll finish this PR and integrate it.

[stephengold]

I believe this is ready to be integrated now.