Resolve #3503356 "Fatal error when"

src/Form/RealmsConfigForm.php

src/Form/SettingsForm.php

src/Plugin/Field/DisplayNameFieldItemList.php

src/Plugin/Validation/Constraint/UserMailUnique.php

src/Plugin/Validation/Constraint/UserMailUniqueValidator.php

src/Routing/ImmutableTrustedRedirectResponse.php