Hi Justin,
I’ve been digging in to this, and filed bug report UE-39106. It looks like what’s happening is that when we reparent your ItemSlot_UI Userwidget, the slate resources for its hierarchy are supposed to be release and recreated. We release them, but we still have a reference to them since the instigating OnClick event came from the slate widget. We see that slate widgets already exist and end up pointing the widgets in the hierarchy back to the now released resources instead of rebuilding them. This means the UWidgets now don’t have their underlying SWidgets, and many of the widget nodes wont work.
I’m working on a proper fix, but in the meantime you can add a Delay(0) node between your click event and reparenting of the widget. This will defer the reparenting so that there are no longer any references to the slate widgets. I’ll post back here once I have a better solution for you.
Best,
Cody