Widgets that are pooled are given a pointer to a layout cache, if the layout cache dies, the pointer is bogus. Going to need to move over to using weak pointers to avoid the crash, will add some performance cost, but will be safer.
Place a widget into an invalidation panel.
Paint it for a few frames.
Remove the widget from the invalidation panel
Delete the invalidation panel
Invalidate the removed widget, the pointer to the layout cache is bogus now so it crashes.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34185 in the post.