Fix - Removing disabled filters

I got a weird error with the filter bar where one of the filters in my app state was null, which caused `onlyDisabled` to throw. I also noticed while checking that out that the app state was not saving filter changes when only removing disabled filters. This PR fixes both by:

- Using `deepGet` in `onlyDisabled`
- Saving `appState` when only removing disabled filters

I also added a simple test to ensure that no exceptions are thrown when the filter list contains `null`.