Difference widget loses selected column names after saving and reopening workflow file (again)

[wvdvegte]

Timeseries version

0.5.1

Orange version

3.33

Expected behavior

After saving and reopening the .ows file, the Difference widget should check if the selected columns still exist, and keep those selected columns that are still available.

Actual behavior

After saving and reopening the .ows file, the Difference widget has forgotten which columns were selected. These have to be manually reselected.

Steps to reproduce the behavior

Open attached workflow, select one or both columns in Difference, save the file, close it and re-open it. The selected columns are no longer selected

Additional info (worksheets, data, screenshots, ...)

This is a partial return of #75. Supposedly, it was resolved in #79

Difference bug.ows.zip