Do not crash when joint SIDs cannot be resolved. OpenCOLLADA cannot resolve joint SIDs connected to skin if the <skeleton> tag is missing. Page 148 of COLLADA spec 1.4.1 states: "<skeleton> elements define where to start the sid lookup". So I think this is a bug in "FBX COLLADA exporter" which did not add the <skeleton> tag. The problem can be fixed by manually adding the <skeleton> tag into <instance_controller>.