Removing OAuth2 integration used by incoming mail server integration makes Jira return 500 error for every page

Issue Summary

Problem related to new method of authentication of incoming mail servers.

Steps to Reproduce

1. Create new OAuth 2.0 integration in Jira (Administration > System OAuth 2.0)
2. Create new incoming mail server integration (Administration > Incoming mail > Add new Mail server)
3. Select integration from Step 1 as authorization method in new mail server integration.
4. Remove OAuth 2.0 integration from Step 1.
5. Restart Jira or wait for cache invalidation (around 30 minutes). Removing an another mail server configuration also triggers cache invalidation

Expected Results

Jira work without problem.

Actual Results

Every page in Jira returns 500 errors due to cache creation problems

Workaround

Remove from database table `MAILSERVER` entry for mail server associated with removed OAuth2 integration