Add None check to expunge errors

I sometimes get errors in my log, which occur when there is no view.

```python
Traceback (most recent call last):
  File "./SublimeBlockCursor.py", line 43, in on_command_mode_change
  File "./SublimeBlockCursor.py", line 22, in on_selection_modified
AttributeError: 'NoneType' object has no attribute 'settings'
```