Can't delete Behaviour mapping with a null Issue Type

Overview

**If you have a Behaviour that is mapped to an issue-type that has been deleted in your instance, an error is thrown when you try deleting that mapping.

Steps to Reproduce:

• In a clean instance of JIRA, create a new project.
• Next, navigate to the Issue Type configuration page and create a new issue-type called "Delete Me."
• Now, head over to the SR Behaviour's Admin page, create a new test behaviour, and map it to your new project and the issue-type that you just created.
• From here, go back to the Issue Type configuration page and delete the issue-type that you created called "Delete Me."
• Finally, head back over to the Behaviour's page and try to delete the mapping that you had just created. You should see an error similar to:
   

As well as an error in the Browser console: