Protect few possible access to window that may be triggered by callback

[Czaki]

References and relevant issues

Hardening suggested in #7560 (comment)

Description

Protect few qt slots with check if qt object still exists

This is a temporary solution. We should move callbacks that require Qt into Qt object

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.85%. Comparing base (c81efa9) to head (ddc5857).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
napari/_qt/qt_main_window.py	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7565      +/-   ##
==========================================
- Coverage   92.88%   92.85%   -0.04%     
==========================================
  Files         630      630              
  Lines       59066    59071       +5     
==========================================
- Hits        54864    54849      -15     
- Misses       4202     4222      +20     

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[psobolewskiPhD]

Real improvement to the docs build logs:
https://app.circleci.com/pipelines/github/napari/napari/8179/workflows/54b440e6-d359-4697-b553-543b5e1bf5d9/jobs/7982
Helps address: napari/docs#596