Dash labels are children of the main uiGroup and so could be destroyed
before their logical-owner DashItemContainer during shutdown.
This implies that we could destroy them twice. To avoid this, unset them
when destroyed.
This is mostly visible when using dash-to-dock, but it's still
technically possible with upstream code:
** Message: 19:57:49.847: Shutting down GNOME Shell
(gnome-shell:2788214): Gjs-CRITICAL **: 19:57:49.933: Object St.Label
(0x55b33668eab0), has been already disposed — impossible to access it.
This might be caused by the object having been destroyed from C code using
something such as destroy(), dispose(), or remove() vfuncs.
== Stack trace for context 0x55b3345fd3d0 ==
#0 7ffeabd810d0 b /data/GNOME/gnome-shell/js/ui/dash.js:86
#1 55b335b62f88 i /data/GNOME/JHBUILD_HOME/.local/share/gnome-shell/extensions/dash-to-dock@micxgx.gmail.com/docking.js:487
#2 7ffeabd838f0 b self-hosted:1121
#3 55b335b62ec8 i /data/GNOME/gnome-shell/js/ui/layout.js:240
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2739>
8bf06bfc9f