Upgrading UPM to 2.19.X breaks Add-On and Manage Add-on page

Issue Summary
Currently, when some users upgrade their UPM version to 2.19.X, their Manage Add-on page and at time Add-ons page do not show the contents expected to be in them.

Steps to Reproduce

• There are 3 scenarios this problem occurs.
  • Upgrade JIRA UPM plugin from 2.18.X to 2.19.x.(Most common)
  • Fresh install of JIRA with the latest UPM version.
  • Upgrade JIRA and UPM to latest version.
• After one of the scenarios mentioned above, user should proceed to the manage add-on page

Expected Result
The JIRA will show all plugins the user has installed.

Actual Result
JIRA would show the manage Add-on page however the Contents (installed plugins) would not show up and the section for it is broken.
Also, the button to Upload Add-on would be missing.

Workaround
Downgrade UPM to version 2.18.4 using the manual method as detailed below:

• Download Universal Plugin Manager
• Stop JIRA
• Go to ..\JIRA HOME\plugins\.osgi-plugins\transformed-plugins
• Delete the plugin atlassian-universal-plugin-manager-plugin-XXXXXXXXXX.jar (Where XXX is the version number however just search for Universal plugin Manager)
• Go to ..\JIRA HOME\plugins\installed plugins
• Delete the plugin atlassian-universal-plugin-manager-plugin-XXXXXXXXXX.jar (Where XXX is the version number however just search for Universal plugin Manager)
• Go to $JIRA_INSTALL/atlassian-jira/WEB-INF/bundled-plugins/
• Delete the plugin atlassian-universal-plugin-manager-plugin-XXXXXXXXXX.jar (Where XXX is the version number however just search for Universal plugin Manager)
• Place the downloaded JAR file in the installed plugins folder $JIRA_INSTALL/atlassian-jira/WEB-INF/bundled-plugins/
• Restart JIRA.
  This would install plugin in JIRA.

Resolution

Upgrade to UPM 2.20.2