[FIX] OWFeatureStatistics: Don't attempt to sort when no data on input

[pavlin-policar]

Issue

An issue popped up on sentry where Feature Statistics was crashing. This can be reproduced in the following way:

1. Open a workflow and connect File with some data to Feature Statistics and save the workflow
2. Remove/rename the data file in question so that the File widget can no longer find it
3. Re-open the workflow in Orange → Feature statistics crash

Description of changes

Fixes the bug. I have no idea how to test this, or indeed, how this even happened because this issue never occurred when disconnecting data. In both cases, the input signal data was set to None.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3449 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3449      +/-   ##
==========================================
- Coverage   83.27%   83.27%   -0.01%     
==========================================
  Files         365      365              
  Lines       64305    64305              
==========================================
- Hits        53549    53547       -2     
- Misses      10756    10758       +2

[lanzagar]

So __restore_sorting does nothing if self.data is None. In that case maybe it should be called only when self.data is not None (in the set_data method above)? Is the same true for __restore_selection?

[pavlin-policar]

I thought about putting the None check in set_data, but decided to put it here instead to keep the __restore_* methods consistent. The other methods properly handle the case where there is no data, only this one fails. It seemed cleaner to just make them all work.