This prevents _sync() being called after actor destruction (since actor
destruction became more reliable in mutter@3d94c7cc2) and so eliminates
this shutdown error:
```
(gnome-shell:35197): Gjs-CRITICAL **: 16:31:02.769: Object .Gjs_ui_calendar_Placeholder (0x559ed6e547e0), has been already disposed — impossible to set any property on 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 0x559ed6022310 ==
#0 559ed783c5a8 i resource:///org/gnome/shell/ui/calendar.js:1012 (31955be5fc90 @ 148)
#1 7ffef8f38230 b self-hosted:1178 (32af8f6b0c40 @ 454)
#2 559ed783c518 i resource:///org/gnome/shell/ui/popupMenu.js:806 (31955be18ce0 @ 52)
#3 559ed783c488 i resource:///org/gnome/shell/ui/popupMenu.js:954 (31955be190b0 @ 168)
#4 559ed783c3f8 i resource:///org/gnome/shell/ui/panelMenu.js:189 (31955be88150 @ 41)
#5 7ffef8f3de60 b self-hosted:1178 (32af8f6b0c40 @ 423)
```
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2429>
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