Traceback (most recent call last):
  File "/usr/bin/update-manager", line 117, in <module>
    app.start_update()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 215, in start_update
    self.start_available()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 233, in start_available
    self.refresh_cache()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 396, in refresh_cache
    self.update_list.update(self.cache, eventloop_callback=iterate)
  File "/usr/lib/python3/dist-packages/UpdateManager/Core/UpdateList.py", line 471, in update
    self.distUpgradeWouldDelete = cache.saveDistUpgrade()
AttributeError: 'NoneType' object has no attribute 'saveDistUpgrade'