Need to make sure we support nullptr entries or make sure we do not create such null entries.
Create a fbx that have two UV channels named UVChannel_1 and UVChannel_3. The FBXUVs import helper will create a set with [UVChannel_1, nullptr, UVChannel_3]and will assert after
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-133118 in the post.