Filter fix

This PR fixes the following issues for the dropdown list filter:

1. Dropdown list content is cached.
2. Submitting filter form with empty list renderers an error and causing the index page to load the error instead of the model data

### Todos

- [x] Removed caching for filter content
- [x] Add validation to check submitted filters are not defined

### Review

@smebberson This is ready for review!