Fixed a bug that caused an error when an invalid data was added to the table viewer

[astrofrog]

and the table viewer was then automatically closed. The error was:

AttributeError: 'NoneType' object has no attribute '_data'

[codecov]

Codecov Report

Merging #2103 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2103      +/-   ##
==========================================
+ Coverage   87.99%   88.03%   +0.04%     
==========================================
  Files         245      245              
  Lines       22131    22132       +1     
==========================================
+ Hits        19475    19485      +10     
+ Misses       2656     2647       -9

Impacted Files	Coverage Δ
glue/viewers/table/qt/data_viewer.py	96.74% <100%> (+0.01%)	⬆️
glue/plugins/dendro_viewer/data_factory.py	97.72% <0%> (-2.28%)	⬇️
glue/app/qt/application.py	79.57% <0%> (+1.06%)	⬆️
glue/__init__.py	70.37% <0%> (+3.7%)	⬆️

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