Designer does not completely undo adding a Dock Widget

Open attached .ui file.

Drag and drop a dock widget to the bottom of the form.

Notice that dock widget is added in Object Inspector and in the form.

Select Undo via the shortcut or Edit menu.

Notice that the dock widget is removed from the form, but still listed in the Object Inspector

Also attached is the error received when attempting operations on the dick widget in the Object Inspector.