## 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_old_style_properties(self):
        filter = Filter(data={"properties": {"$browser__is_not": "IE7", "$OS": "Mac",}})
        self.assertEqual(filter.properties[0].key, "$browser")
        self.assertEqual(filter.properties[0].operator, "is_not")
        self.assertEqual(filter.properties[0].value, "IE7")
        self.assertEqual(filter.properties[0].type, "event")
        self.assertEqual(filter.properties[1].key, "$OS")
        self.assertEqual(filter.properties[1].operator, None)
        self.assertEqual(filter.properties[1].value, "Mac")
    def test_to_dict(self):
        filter = Filter(
            data={
                "events": [{"id": "$pageview"}],
                "display": "ActionsLineGraph",
                "compare": True,
                "interval": "",
                "actions": [],
                "date_from": "2020-01-01T20:00:00Z",
            }
        )
        self.assertCountEqual(list(filter.to_dict().keys()), ["events", "display", "compare", "insight", "date_from"])
class TestSelectors(BaseTest):