[stable-3.9] prevent crash by resetting common pointer after deleting gobject menu

[backportbot-nextcloud]

Backport of #5845

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #5850 (df4eeb6) into stable-3.9 (499c9f2) will increase coverage by 0.07%.
The diff coverage is 85.29%.

Additional details and impacted files

@@              Coverage Diff               @@
##           stable-3.9    #5850      +/-   ##
==============================================
+ Coverage       60.36%   60.43%   +0.07%     
==============================================
  Files             143      145       +2     
  Lines           18563    18609      +46     
==============================================
+ Hits            11205    11246      +41     
- Misses           7358     7363       +5     

Impacted Files	Coverage Δ
src/common/checksums.h	0.00% <ø> (ø)
src/common/checksums.cpp	73.57% <60.00%> (-6.00%)	⬇️
src/common/checksumcalculator.cpp	87.64% <87.64%> (ø)
src/common/checksumcalculator.h	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[nextcloud-desktop-bot]

AppImage file: nextcloud-PR-5850-df4eeb6368caa4b670e49fb076dd806babf68fc1-x86_64.AppImage

To test this change/fix you can simply download above AppImage file and test it.

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