UMG: crash when a namedslot has content in a widget blueprint and its instance

workaround: open WBP_A and remove its namedslot content to avoid having two contents in the same namedslot.

1. Open QAGame
2. Create a new widget blueprint named WBP_A. In the widget tree, add a canvas panel and a namedslot. 
3. Create another widget blueprint named WBP_B. In the widget tree, add a canvas panel and an instance of WBP_A.
4. From WBP_B, add a dynamic entry box to the namedslot of WBP_A. Compile and save.
5. Then go back to WBP_A and add a dynamic entry box to its namedslot. Compile and save.
6. Restart the editor and open WBP_B
7. Observe: crash