App is not responding

Summary

Most add-ons that are integrated into the issue details view won't load when you view an issue from the side panel. The behaviour is the same for all add-ons for any type of project in Jira.

Steps to Reproduce

1. Install an addon on your Jira (in this example tempo timesheets was used)
2. Go to your project's issues panel and select a random issue to view (if it doesn't happen on the first try select another random issue) 

Expected Results

On the side view, the issue details will appear along with its fields and the add-on integrated amongst them.

Actual Results

The add-on will not load, it takes a few seconds to attempt to load and then fails with this message:

 App is not responding. Wait or cancel?

Workaround

There is no actual workaround besides refreshing the page or clicking again on the issue, then the issue will reload with the add-ons loaded.