Issue #2015721 by dawehner: Fixed Adding a role to a user throws a php error.

...odules/user/lib/Drupal/user/Plugin/Action/AddRoleUser.php

...les/user/lib/Drupal/user/Plugin/Action/RemoveRoleUser.php

.../modules/user/lib/Drupal/user/Plugin/Core/Entity/User.php

core/modules/user/lib/Drupal/user/Tests/UserEntityTest.php

core/modules/user/lib/Drupal/user/UserBCDecorator.php

core/modules/user/lib/Drupal/user/UserInterface.php

...tests/Drupal/user/Tests/Plugin/Action/AddRoleUserTest.php

...ts/Drupal/user/Tests/Plugin/Action/RemoveRoleUserTest.php