An anim composite will freeze the editor if one of its animation segments has a a notify and a length of 0 or less. Debugging shows that the engine is stuck inside UAnimSequenceBase::GetAnimNotifies. This is likely because UAnimComposite::IsNotifyAvailable() does not check for a sequence length greater than 0.
The editor will freeze indefinitely
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-71846 in the post.