Avoid unecessary warnings

[astrofrog]

Avoid warning when closing empty tab, and avoid additional warnings when closing a tab with open viewers.

[codecov]

Codecov Report

Merging #1890 into master will decrease coverage by <.01%.
The diff coverage is 54.54%.

@@            Coverage Diff            @@
##           master   #1890      +/-   ##
=========================================
- Coverage   85.51%   85.5%   -0.01%     
=========================================
  Files         235     235              
  Lines       20892   20896       +4     
=========================================
+ Hits        17865   17868       +3     
- Misses       3027    3028       +1

Impacted Files	Coverage Δ
glue/viewers/common/qt/base_widget.py	73.38% <100%> (-0.22%)	⬇️
glue/app/qt/application.py	77.89% <50%> (+0.02%)	⬆️
glue/viewers/common/state.py	84.21% <0%> (-15.79%)	⬇️
glue/core/data_factories/fits.py	89.06% <0%> (+0.78%)	⬆️
glue/utils/qt/helpers.py	55.55% <0%> (+1.11%)	⬆️

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