fixed bug in sysmon widget loading exception

```
Traceback (most recent call last):
  File "/home/duncan/repos/genesis/genesis/core/application.py", line 164, in dispatcher
    self.start_response)
  File "/home/duncan/repos/genesis/genesis/api/urlhandler.py", line 89, in url_handler
    return handler(req, start_response)
  File "/home/duncan/repos/genesis/genesis/plugins/core/root.py", line 294, in handle_generic
    main = self.main_ui()
  File "/home/duncan/repos/genesis/genesis/plugins/core/root.py", line 47, in main_ui
    templ.append('main-content', self.selected_category.get_ui())
  File "/home/duncan/repos/genesis/genesis/plugins/sysmon/main.py", line 53, in get_ui
    self.fill('l', self._mgr.list_left(), ui, 'cleft')
  File "/home/duncan/repos/genesis/genesis/plugins/sysmon/main.py", line 45, in fill
    self.app.log.error('System Monitor Widget failed to load '+w.title+': '+str(e))
TypeError: cannot concatenate 'str' and 'NoneType' objects
```