Custom filter table: trying to add a filter list header does not trigger an error

Issue

In current release 2018-5 if the custom filters table is empty, and one attempts to add a filter list header as the first filter (for example [abp]), nothing happens in the UI. There are also no errors in the console. If the filter list header is attempted to be added later, when the filter table already has some other filters in it, the error is shown in the filter table's footer (as it should be).

Reproduced in latest versions of Chrome, Opera and Firefox (did not try older browser versions)

Reproduction steps

1. Open the Options page and navigate to the Advanced tab
2. Make sure the custom filters table is empty
3. Try adding the filter list header, for example [test]

--> notice that nothing happens in the UI

Expected behavior

Trying to add a custom filter that has the format of a header should always trigger the unexpected_filter_list_header error message: 'Filter list headers aren't allowed here' (https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/blob/release-2019-1/locale/en_US/filter-validation.json#L46)