Originally reported here: https://udn.unrealengine.com/questions/562047/linked-layer-animation-blueprint-proxy-missing-ske.html
When linking an anim layer during update, we get either an ensure or crash (missing skeleton) depending on the structure of the graph.
Ensure message: "PreUpdate isn't called. This could potentially cause other issues."
This seems like a valid code path to support. Seems we'd either need to queue the link (introducing a frame delay?) or do some sort of fixup.
We haven't encountered this internally because we've so far only been linking layers from gameplay code.
Call LinkAnimClassLayers() from NativeUpdateAnimation or BlueprintUpdateAnimation.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-90574 in the post.