[FIX] Fix crash in listview if labels are changed before calling __setitem__

[janezd]

Issue

Confusion matrix, and possibly other widgets, may change the items in the listview before triggering ControlledList.__setitem__. __setitem__ already took care about this by checking whether self.listbox(item) returned None. This apparently doesn't always work since one user got

        Traceback (most recent call last) 
          File "E:\\ORANGE\\Python34\\lib\\site-packages\\Orange\\canvas\\scheme\\widgetsscheme.py", line 822, in process_signals_for_widget
            handler(*args)
          File "E:\\ORANGE\\Python34\\lib\\site-packages\\Orange\\widgets\\evaluate\\owconfusionmatrix.py", line 270, in set_results
            self.selected_learner[:] = prev_sel_learner
          File "E:\\ORANGE\\Python34\\lib\\site-packages\\Orange\\widgets\\gui.py", line 2062, in __setitem__
            unselect(i)
          File "E:\\ORANGE\\Python34\\lib\\site-packages\\Orange\\widgets\\gui.py", line 2050, in unselect
            item = self.listBox.item(i)
        RuntimeError: wrapped C/C++ object of type OrangeListBox has been deleted

Description of changes

Besides checking for None, the method now also catches the exception.

Includes

• Code changes

[lanzagar]

This is probably hard to test, since we don't know how to reproduce the error, right?
I am guessing RunTimeError is a typo of RuntimeError.

[janezd]

I changed RunTimeError to RuntimeError (apparently Python notices this only when trying to handle the exception).

I tried to reproduce the error with a test, but haven't gotten anywhere, so --- no tests. This happens in ControlledList, which we use in list views that do not use models. IMHO, we should migrate all list views to models anyway.

[codecov-io]

Current coverage is 89.20% (diff: 100%)

Merging #1825 into master will not change coverage

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