mate-appearance-properties or mate-tweak causes flood of ~/.xsession-errors with Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed

[Norbert]

Steps to reproduce:
0. Open terminal with `tail -f ~/.xsession-errors`
1. Open mate-appearance-properties , switch theme, change background
2. Open mate-tweak, change window manager to "Marco (no compositor)" or back to "Marco (adaptive compositor)"
3. Go back to mate-appearance-properties , switch theme, change background

Expected results:
* theme, background and window manager switched, no errors shown in ~/.xsession-errors .

Actual results:
* the ~/.xsession-errors have serious flood with messages like

```
Window manager warning: Log level 128: unsetenv() is not thread-safe and should not be used after threads are created
Window manager warning: Screen 0 on display ":0" already has a window manager; try using the --replace option to replace the current window manager.
Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed
Window manager warning: Log level 128: unsetenv() is not thread-safe and should not be used after threads are created
Window manager warning: Screen 0 on display ":0" already has a window manager; try using the --replace option to replace the current window manager.
Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed
...
```

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: mate-tweak 20.04.0-2
ProcVersionSignature: Ubuntu 5.8.0-18.19-generic 5.8.4
Uname: Linux 5.8.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu45
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: MATE
Date: Sun Sep 13 18:42:58 2020
InstallationDate: Installed on 2020-09-13 (0 days ago)
InstallationMedia: Ubuntu-MATE 20.10 "Groovy Gorilla" - Alpha amd64 (20200913)
PackageArchitecture: all
SourcePackage: mate-tweak
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1895460

[Norbert (nrbrtx)]

Fixed in impish.

[Simon Butcher (sbutcher)]

i still see this in jammy

Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed
Window manager warning: Log level 128: unsetenv() is not thread-safe and should not be used after threads are created
Window manager warning: Screen 0 on display ":0" already has a window manager; try using the --replace option to replace the current window manager.
Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed
Window manager warning: Log level 128: unsetenv() is not thread-safe and should not be used after threads are created
Window manager warning: Screen 0 on display ":0" already has a window manager; try using the --replace option to replace the current window manager.
Window manager warning: Log level 8: g_main_loop_is_running: assertion 'loop != NULL' failed
Window manager warning: Log level 128: unsetenv() is not thread-safe and should not be used after threads are created