Error in web UI when an unexpected entry is encountered during generation of a drop-down list

[plettich]

In v3.8 we introduced drop-down lists instead of text fields for fixed input lists.
But in the case that an entry was already in the text field list but not in the possible values for the drop-down list, the UI failed with an error and stopped displaying the rest of the page.
This happens for example when the challenge_response action contained email or sms which are inherently challenge-response token and thus not selectable as possible values in the drop-down list.

[plettich]

Closed with #3938