Sprint Burndown gadget is empty in Jira 8.8.0

Issue Summary

After upgrading to Jira 8.8.0 (even fresh install), the Sprint burndown gadget is blank.

Steps to Reproduce

1. Add the Sprint Burndown gadget in the dashboard
2. Select any existing board in the instance and then save.

Expected Results

The gadget will show the graphical sprint burndown chart data

Actual Results

The gadget is empty. Notice the browser console (on Chrome) as it shows:

Uncaught TypeError: Cannot read property 'getSelectedItem' of undefined

Firefox's browser console can display a different message:

TypeError: e.sprintPicker is undefined

Workaround

Currently, there is no known workaround for the gadget to work. However, the same content/functionality is present on the Burndown Chart Report, which can be used as a replacement until the gadget will be fixed.

Tested in Jira 8.6.1 and the gadget works as expected.