## 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
import json
from typing import Any, Callable, Optional
from dateutil.relativedelta import relativedelta
from django.db.models import Q
from django.utils import timezone
from freezegun.api import freeze_time
from posthog.models import Cohort, Element, Event, Filter, Organization, Person, Team
from posthog.queries.base import properties_to_Q
from posthog.test.base import BaseTest
class TestFilter(BaseTest):