TestAfter moving a widget in the designer tab and then using Ctrl+Z to undo this will cause the widget that was moved to be deleted.
Unreal Engine 4.7 Preview
4.7 Preview - Known Issues List
[Bug] UMG - Version 4.7 - Preview 6 - CTRL-Z (Undo Move) after moving a widget deletes the widget
Head over to the existing Questions & Answers thread and let us know what's up.