Error when replacing missings

[loucasal]

Steps to reproduce:

1. Start creating a new question.
2. Insert an existing response domain.
3. From the missings dialog, select a first set of missing codes.
4. Click 'X' to clear the search and select a different set of missings.
5. Fill in the codes and submit.

Result:

1. Error code 500 (screenshot attached).
2. The missings appear at the bottom of the page, but not in the list of codes and the item cannot be submitted. The whole response domain needs to be removed and readded.