OWTable crashes when loading a missing file with selected data

[pavlin-policar]

Orange version

master

Issue

A similar issue to what is described in #3449. Loading a workflow where the source data file can no longer be found causes crashes. One would expect that in this case, None would be sent to inputs (just like removeing a widget signal on canvas), however it seems that this is not the case?

I've gone through most of the widgets in the Data and Visualize section and haven't been able to reproduce this bug, but it might be hiding in other widgets as well.

Traceback (most recent call last):
  File "/home/pavlin/dev/orange3/Orange/canvas/scheme/widgetsscheme.py", line 1073, in process_signals_for_widget
    handler(*args)
  File "/home/pavlin/dev/orange3/Orange/widgets/data/owtable.py", line 535, in set_dataset
    self.set_selection()
  File "/home/pavlin/dev/orange3/Orange/widgets/data/owtable.py", line 775, in set_selection
    model = view.model()
AttributeError: 'NoneType' object has no attribute 'model'

Steps to reproduce the behavior

1. Open a workflow and connect File with some data to a Table, select some rows 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 → crash