Fix jipt not working when component has been removed.

Summary:
On http://translate.khanacademy.dev/preview/content/items?exercises=addition-word-problems-within-10#x6231985cdcc6b096
we were throwing errors when the jipt callback occurs, but the component is already gone. This
could happen if you open up the jipt edit window on one exercise item and then go to the next item.

Test Plan:
Go to: http://translate.khanacademy.dev/preview/content/items?exercises=addition-word-problems-within-10#x6231985cdcc6b096
Click on the jipt edit icon.
Click on a new assessment item.
Click on the jipt edit icon (and see no js errors occur and current text in the edit window is the expected one).

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D9359