It's not possible to assign any User or group to a Role.

Steps to reproduce:
1. Go to Admin -> Roles
2. Create a new Role
3. Go to Assignments tab and choose Assign users/groups
4. Select "Administrator user"
5. Click "Create"

Expected:
User is assigned to the Role

Actual:
JS error in console:

Uncaught TypeError: Cannot read property 'click' of null
    at HTMLButtonElement.trigger (0bfb931.js:952)