[FIX] FreeViz: 2 issues when no data

[jerneju]

Issue

Widget crashes when there is no data and

1. Radius slider is moved
2. Class density, etc. is changed

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2780 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2780      +/-   ##
==========================================
+ Coverage   76.09%   76.09%   +<.01%     
==========================================
  Files         338      338              
  Lines       59723    59735      +12     
==========================================
+ Hits        45444    45455      +11     
- Misses      14279    14280       +1

[janezd]

_update_graph is called from reset_graph_data and update_density. The former already checks that graph.data is not None. You could add this check to the latter as well instead of having it in _update_graph, and just move the assert in _update_graph to the top.

[jerneju]

Done.

[lanzagar]

Not in the scope of this PR (which fixes bugs and we want in the 3.8 release), but maybe for future refactoring

1. assert not self.plotdata is None should be assert self.plotdata is not None
2. I fail to see why the above asserts are needed at all (plotdata is never None, even after initialization)

Back to the PR - the fixes work for me and look ok.