[FIX] Ontology: Fix 'remove word' bug

[VesnaT]

Issue

Removing the bar word from the above ontology causes an error:

Traceback (most recent call last):
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 803, in __on_ontology_data_changed
    self.__set_current_modified(self.CACHED)
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 899, in __set_current_modified
    ontology = self.__ontology_view.get_data()
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 324, in get_data
    return _model_to_tree(self.__root, selection, with_selection)
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 57, in _model_to_tree
    _model_to_tree(item.child(i), selection, with_selection)
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 57, in _model_to_tree
    _model_to_tree(item.child(i), selection, with_selection)
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 57, in _model_to_tree
    _model_to_tree(item.child(i), selection, with_selection)
  File "/Users/vesna/orange3-text/orangecontrib/text/widgets/owontology.py", line 55, in _model_to_tree
    for i in range(item.rowCount()):
AttributeError: 'NoneType' object has no attribute 'rowCount'

Includes

• Code changes
• Tests
• Documentation

[PrimozGodec]

Thanks for the fix. :)