## Bug description

The following filter fails with a 500 internal error.

![2021-02-04_12-07](https://user-images.githubusercontent.com/148820/106877644-a9f4ce80-66e1-11eb-8320-87bb213bdde2.png)


## Expected behavior

Error is shown to user about invalid regular expression.

Note no errors shown on the page as well.

## How to reproduce

1. Go to events
2. Add filter "Current URL matches regular expression ?*"
3. See infinite loading screen / error

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: _please provide_

## Additional context

Note this can also happen when filtering by events.

#### *Thank you* for your bug report – we love squashing them!

posthog/models/filters/test/test_filter.py
def test_selectors(self):
        event1 = Event.objects.create(
            team=self.team,
            event="$autocapture",
            elements=[Element.objects.create(tag_name="a"), Element.objects.create(tag_name="div"),],
        )
        event2 = Event.objects.create(team=self.team, event="$autocapture")
        filter = Filter(data={"properties": [{"key": "selector", "value": "div > a", "type": "element"}]})
        events = Event.objects.filter(properties_to_Q(filter.properties, team_id=self.team.pk))
        self.assertEqual(events.count(), 1)
def property_to_Q_test_factory(filter_events: Callable, event_factory, person_factory):
    class TestPropertiesToQ(BaseTest):
        def test_simple(self):
            event_factory(team=self.team, distinct_id="test", event="$pageview")
            event_factory(
                team=self.team, distinct_id="test", event="$pageview", properties={"$current_url": 1}
            )  
            event_factory(
                team=self.team, distinct_id="test", event="$pageview", properties={"$current_url": {"bla": "bla"}}
            )  
            event_factory(
                team=self.team,
                event="$pageview",
                distinct_id="test",
                properties={"$current_url": "https://whatever.com"},
            )
            filter = Filter(data={"properties": {"$current_url": "https://whatever.com"}})
            events = filter_events(filter, self.team)
            self.assertEqual(len(events), 1)
        def test_incomplete_data(self):