Fatal error when adding non-existent user to a site

[duck_]

Attempting to use site-users.php in the network admin to add a non-existent user to a site results in a fatal error since the call to get_user_by() will return false.

Fatal error: Call to a member function exists() on a non-object in wp-admin/network/site-users.php on line 89

Introduced in [23974]

[SergeyBiryukov]

In 24509:

Avoid a fatal error when adding a non-existent user to a site. props duck_. fixes #24460.