Code review comment for lp:~phill-ridout/openlp/pathlib2

Trunk is fine

Advanced Tab
Traceback (most recent call last):
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/mainwindow.py", line 810, in on_settings_configure_iem_clicked
    self.settings_form.exec()
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/settingsform.py", line 70, in exec
    self.insert_tab(self.general_tab)
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/settingsform.py", line 89, in insert_tab
    log.debug('Inserting {text} tab'.format(text=tab_widget.tab_title))
AttributeError: 'NoneType' object has no attribute 'tab_title'

Theme Edit with image
Traceback (most recent call last):
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/thememanager.py", line 326, in on_edit_theme
    self.theme_form.exec(True)
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/themeform.py", line 279, in exec
    self.set_defaults()
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/themeform.py", line 109, in set_defaults
    self.set_background_page_values()
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/ui/themeform.py", line 322, in set_background_page_values
    self.image_path_edit.path = path_to_str(self.theme.background_filename)
  File "/home/tim/Projects/OpenLP/openlp/pathlib2/openlp/core/common/path.py", line 37, in path_to_str
    raise TypeError('parameter \'path\' must be of type Path or NoneType')
TypeError: parameter 'path' must be of type Path or NoneType