"NullError: method not found: 'buffer' on null" when validating glTF

[bghgary]

Repro:

1. Take this model SittingBox.zip and put it on the glTF-Validator web page.

This original repro is trying to validate a glb in the BabylonJS sandbox that came from here: https://github.com/mrdoob/three.js/blob/master/examples/models/gltf/SittingBox.glb. The zip file with the glTF file was extracted from the glb without modification.

If I set validateAccessorData to false, then it doesn't repro.

[lexaknyazev]

I've fixed crash internally, will open a PR with a couple more updates following recent discussions.

Btw, this asset shows a new corner case. A non-GLB file which doesn't have buffer.uri defined. Spec doesn't seem to address this one.