This is a known issue (UE-23217) that has been reported and sent to the developers for further consideration. I have provided a link to the original issue below. Please visit that thread for future updates. I have also provided the workaround available at the link provided.
Problem now happens on version 4.19.2. Same workaround works: reset layout. It seems to happen when you try to bind a function to a UI widget by selecting from a menu, but accidentally move the mouse cursor off of the menu while it is up.