Jenkins returns NPE when saving a job with no JIRA instance selected in the build step

This tipically happens when you install the plugin for the first time and the user tries to save a job with no JIRA instance selected in the build step.

Steps to reproduce this bug:

1. Uninstall the jenkins plugin
2. Restart jenkins server
3. Delete the ServerConfig.xml file under the jenkins installation home
4. Turn on the jenkins server
5. Install the plugin
6. Create a freestyle project and configure the job
7. Configure an Xray build step, for instance Xray:Import results
8. Save the job
9. A Null pointer is returned