Filter value popover in streams overview closes when pressing shift.

## Description

You can apply filters for the streams overview. For example the list for the index set filter allows you to select multiple index set while pressing shift. This currently no longer works, because the filter value popover closes when pressing shift.

## Steps to reproduce

1. Go to streams overview
2. Click create filter button
3. Select any filter type
4. Press shift

* Graylog Version: 6.1