Fix a crash with the Changes API when unable to load a field type.

Review Request #7463 — Created June 26, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
9ec235b...

Reviewers

If a change was made to a review request with a pluggable field, and
that field was no longer available (due to the extension no longer being
enabled), the code would fail, resulting in an HTTP 500.

We now check to make sure the field is there before including the
serialized contents in the payload.

I was hitting this problem locally. This fixed it.