I encountered this bug in 4.7.1. Just verified that it’s present in 4.7.2 as well.
Working in the UMG designer, copy/cut a widget and paste it into a border that already contains a widget. The entire editor crashes.
(I know a border can only contain a single child widget. Just that it’s really easy, when working with many nested widgets, to paste something on accident into the border and end up with a crash.)