Only a drag context which was created with 'drag_begin' will be
guaranteed to have a source window at all times. Thus, in finalize we
cannot assume we can retrieve a GdkDisplay from the source_window
pointer since it may be NULL. Though, the display is only needed for
contexts created via 'drag_begin' thus we can retrieve it after
checking that is the case.
https://bugzilla.gnome.org/show_bug.cgi?id=749339
27763743b1