[GUI] Fixed crash on MacOS when resetting a greyed dialog parent

[Max-7]

Closes #1538

[codecov]

Codecov Report

Merging #1539 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1539      +/-   ##
==========================================
+ Coverage   87.71%   87.82%   +0.10%     
==========================================
  Files         241      241              
  Lines       22952    22953       +1     
==========================================
+ Hits        20132    20158      +26     
+ Misses       2820     2795      -25     

Impacted Files	Coverage Δ
parsec/core/gui/custom_dialogs.py	80.40% <100.00%> (+0.09%)	⬆️
parsec/core/gui/parsec_application.py	93.47% <0.00%> (-2.18%)	⬇️
parsec/core/fs/remote_loader.py	84.73% <0.00%> (-0.77%)	⬇️
parsec/backend/postgresql/invite.py	95.73% <0.00%> (-0.61%)	⬇️
parsec/core/fs/workspacefs/workspacefs.py	97.32% <0.00%> (-0.30%)	⬇️
parsec/core/gui/workspaces_widget.py	83.22% <0.00%> (+0.21%)	⬆️
parsec/core/fs/userfs/userfs.py	85.96% <0.00%> (+0.43%)	⬆️
parsec/core/gui/greet_user_widget.py	86.62% <0.00%> (+0.58%)	⬆️
parsec/backend/app.py	93.54% <0.00%> (+1.07%)	⬆️
parsec/core/sync_monitor.py	93.22% <0.00%> (+1.19%)	⬆️
... and 1 more

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[Ironicbay]

LGTM !