[BUG] Cinnamon crashes in headless mode

 * Cinnamon 5.4.11
 * Linux Mint 21
 * Intel Iris Plus Graphics 640 / i915 **not used** -> headless mode
 * Default display resolution (headless mode): 1024 x 768
 * 64 bit
 * Attached ~/.xsession-errors

Issue

Linux Mint 21 running in headless mode without a monitor attached with VNC remote access (x11vnc / xrdp) crashes at startup after LightDM login.

Steps to reproduce

If no monitor or no xorg.conf file with dummy monitor settings is present Cinnamon crashes.

Errors (.xsession-errors -> LM21_headless_xsession-errors.txt):

JS LOG: [LookingGlass/info] Monitor 0 not found. Not creating panel
(cinnamon:1420): Gjs-CRITICAL **: 13:44:09.191: JS ERROR: TypeError: layoutManager.primaryMonitor is undefined
start@/usr/share/cinnamon/js/ui/main.js:402:5
@<main>:1:47

Expected behaviour

Earlier version of Cinnamon (≤ 5.2.7) in Linux Mint 20.x "ignored" a missing monitor (-> LM20-3_headless_xsession-errors.txt)
VNC remote access worked flawlessly.
Mint 21 Mate also works without any crashes.

Other information

Use case: Several systems are running in headless mode controlled via VNC at a low display resolution (1024 x 768), but from time to time monitors with much higher resolutions are attached.
An xorg.conf file with dummy monitor settings doesn't work well in such a scenario. So, that's not a proper workaround. (And I'd prefer not to switch to the Mate desktop...)

LM21_headless_xsession-errors.txt
LM20-3_headless_xsession-errors.txt

Start by reproducing the headless startup with no monitor or xorg.conf, then inspect cinnamon/js/ui/main.js around line 402 and the layoutManager.primaryMonitor error in the attached .xsession-errors. Compare the Cinnamon 5.4.11 behavior with the earlier working version; done means Cinnamon starts without crashing and VNC access remains usable in headless mode.