Grouped Window List - Zombies stay alive after being closed

 * Cinnamon version 4.2.4
 * Manjaro 18.0.4
 * AMD 3500U - amdgpu
 * 64 bit
* .xsession-errors gets spammed with these several seconds:
(cinnamon:1285): Cjs-CRITICAL **: 04:03:42.158: JS ERROR: SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data @ /usr/share/cinnamon/js/misc/fileUtils.js line 210 > Function:299
 * syslog - there is none

Issue
When a process gets killed or closed from the thumbnail, sometimes the app either keeps alive or the app icon stays active in the taskbar. (not pinned)

Steps to reproduce
1.) open any application
2.) kill the application with process manager
3.) Profit

Expected behaviour
Apps which get killed or closed by thumbnail should not stay in the taskbar, except when they're pinned. The zombie windows can only be removed by restarting cinnamon and this problem occurs since icingtaskmanager moved to grouped window list. I can't exactly remember how to reproduce but if I close the last window of an application like i.e. a steam chat window and I just started Steam in silent mode before, the app icon also stays in the taskbar and only the label is visible when hovered.

P.S.: If one tries to close the window by using the close-button on the thumbnail (which actually just consists of label and no thumbnail), u will eventually get asked to force quit the process. Which obviously doesn't work, since the process doesn't exist anymore.

Other information
Ha Ha Ha, staying aliiive

Start by reproducing the grouped window list behavior using the process-manager kill and thumbnail-close steps described in the issue. Trace how closed or killed application windows are removed from the taskbar, including the pinned-app exception; the issue is done when non-pinned zombie icons disappear without restarting Cinnamon and the force-quit prompt is no longer offered for already-dead processes.