Add a couple bug fixes to SublimeBlockCursor.py.

Fixed a `NameError`, as `view_is_widget` was defined as a method, but used like a function. So, moved it out of the class, although a smaller fix would have been to just add `self.` to the `view_is_widget` call, in `on_selection_modified()`.

Also, added a None type check, to suppress some errors I saw in the console. The latter fix is slightly different to, but addresses the same issue reported in PR #13. Only just seen that PR, but I applied the patch a while ago, so I'm not exactly sure under what circumstances that error message is produced...