Summary

If you select the button Add flag and comment (board view), it will open a new screen to add a comment, if you select add nothing will happen and you won't be able to cancel the operation, the screen will freeze.
It happens only when the issue on that board is associated to a Service Desk Project.

Steps to Reproduce

1. Create a board that contains issues from a JIRA Service Desk project.
2. Click either Backlog, Active sprints, or Kanban board.
3. Select an issue from a JSD project within the board
4. Click the issue then in question and in the Issue Detail View, select "Add flag and comment".
5. Write the comment and try to add it.

Expected Results

The issue will be flagged and a comment added to the issue.

Actual Results

The screen freezes in JIRA Cloud instances.
In JIRA Server an error is thrown on the screen of

Exception: Uncaught TypeError: Cannot read property 'trim' of undefined  

Workaround

Add the flag and the comment separately (not applicable for Cloud).