Fix issue #78 (Edge split in reshape fails).

When an edge is expunged (as happens when the reshaped edge is split), then the base view calls updateUI() so that the view can remove the deleted entity from any drop-down menus. However, in the case of the arc panel, the arc widget was being deleted. Since the arc widget object was the source of the operation's execution, this was causing memory corruption and crashes. Don't do that.