Make sure change_role property is defined before making a call to it

Summary: Fixt T288

Test Plan:
 - Go to #users
 - Attempt to add a new user
 - Watch console for no signs of the error `undefined`

Reviewers: rjmackay

Reviewed By: rjmackay

Maniphest Tasks: T288

Differential Revision: https://phabricator.ushahidi.com/D175