Web page is broken on adding administrator users to a new group

[ggonzr]

Administrator users are able to edit reports for any group in the application. However, when it is required to add a new group to the application (e.g: #55) and include admin users into this new group, the action is forbidden by the application. In the frontend side, the HTTP 400 response causes an unhandled exception that breaks the page.

Expected Behavior

The exception should be handled properly, allowing the user to close the error modal without freezing completely the page and forcing them to reload the window.

Current Behavior

The exception is not caught and after closing manually the error modal, the page gets frozen.

Steps to reproduce

1. As an administrator user, add yourself to a new group you're not already included.
2. After clicking Add, an HTTP 400 response is sent from the backend server, raising an exception displayed in the web page. Because there's no catch block, the first modal is not closed, and the error modal is displayed.
3. If you close the first modal, the page is still scrollable. As soon as the error modal is closed, the page gets frozen.