Fix crashing when selecting user status and predefined statuses not appearing

[claucambra]

Previously, the signal handler for opening the user status selector dialog would try to trigger the loading of the selected user's user status connector for the user status selector model before the dialog had finished loading. This would cause the loading to never be triggered and the pointer to remain null, which, combined with the lack of a check for a null pointer, would cause a crash.

This PR fixes the user status connector loading behaviour to not be subject to this race condition, and adds checks for the null pointer in the user status selector model.

EDIT: upon testing this PR also fixes an issue where predefined statuses don't appear the first time the user status selector is invoked

[codecov]

Codecov Report

Merging #4616 (ceeb255) into master (5337d4b) will not change coverage.
The diff coverage is n/a.

❗ Current head ceeb255 differs from pull request most recent head 0658d72. Consider uploading reports for the commit 0658d72 to get more accurate results

@@           Coverage Diff           @@
##           master    #4616   +/-   ##
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  Coverage   56.42%   56.42%           
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  Files         138      138           
  Lines       17071    17071           
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  Hits         9632     9632           
  Misses       7439     7439           

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

50.0% Coverage
0.0% Duplication

[nextcloud-desktop-bot]

AppImage file: Nextcloud-PR-4616-0658d7227ee3fea8ef8e3c5dc4da0e4cde59ecc3-x86_64.AppImage

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

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