Copying my comment from T203336:

In T203336#4551532, @Daimona wrote:

1. As specified in the patch commit message, we still need a follow-up to enforce validation on other fields, both client- and server-side. Currently, this includes:
   1. Warning (server-side and client-side): select "Other message" and leave the second field blank. This also produces a PHP notice for undefined offset 0 in ViewEdit (look for $warnMsg = $parameters[0];)
   2. Throttle (client-side): the same as we do server-side in the patch above
   3. Tag (client-side): mostly needs to clearly show that at least one tag is needed. Also, when saving a filter with tag enabled but without touching the tag input field correctly returns an error but an empty tag is added in the field topbar; this shouldn't probably happen, but it's a minor problem.

In short, we need to properly validate (both server- and client-side) action parameters to avoid errors like that. Client-side validation has its own task, T196984.