Restricted Pages dialog do not load if broken users are added to the Restrictions

Issue Summary

If a user that doesn't have an AAID properly set on the Instance is added to the Restrictions of a page, the Restrictions dialog won't load.

 This scenario is uncommon, but it might happen as a result of a Migration (in case the Migration doesn't properly create a user that is part of a Restriction list).

Steps to Reproduce

1. Import a Space that contains a user in this scenario, and that has restrictions granted to this user. 
2. Access the affected page.
3. Click on the restrictions padlock.

Expected Results

The restrictions dialog should load.

Actual Results

The restrictions dialog doesn't load, and an error is displayed:

Workaround

Remove the problematic restriction from the Restricted Pages:

1. Access the Space.
2. Click on the Space Settings, on the sidebar.
3. Navigate to Permissions tab, then Restricted Pages sub tab.
4. The problematic page will display a restriction granted to an Unlicensed user. Click on the padlock next to this entry.