A widget’s location and size can be locked into place. When a widget is selected, clicking Objects->Locking->Lock Location and Size will lock the widget. The command can also be found in the context menu.
Similarly, the location and size can be unlocked by clicking Objects->Locking->Unlock Location and Size.
When a widget is locked, it can still be deleted. However, if unlocked widgets are also in the selection when deleting, the unlocked widgets will be deleted first and a second delete will need to be performed to delete the locked widgets. In other words, to delete locked widgets, only locked widgets can be selected. This applies for Masters placed in the background as well.