Fix segfault on reload with progress bar

[mwestphal]

• Fix a segfault on reload with progress bar
• Feature is not behaving as expected, on reload, the progress bar DO NOT APPEAR but nothing is visible during the reload
• Add progress bar smoke test
• Fix Reloading a file while using a progress bar causes a segfault #761

[codecov]

Codecov Report

Merging #778 (7658acc) into master (8b22935) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 7658acc differs from pull request most recent head 44253f6. Consider uploading reports for the commit 44253f6 to get more accurate results

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
+ Coverage   96.62%   96.69%   +0.06%     
==========================================
  Files         115      115              
  Lines        5776     5777       +1     
==========================================
+ Hits         5581     5586       +5     
+ Misses        195      191       -4     

Impacted Files	Coverage Δ
...brary/VTKExtensions/Rendering/vtkF3DRenderPass.cxx	97.10% <100.00%> (ø)
library/src/loader_impl.cxx	100.00% <100.00%> (+2.83%)	⬆️

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