only allow to add user if mail is set - backend throws error if mail is not set

scm-ui/src/users/components/UserForm.js

@@ -73,7 +73,8 @@ class UserForm extends React.Component<Props, State> {
       this.state.passwordConfirmationError ||
       this.state.displayNameValidationError ||
       this.isFalsy(user.name) ||
-      this.isFalsy(user.displayName)
+      this.isFalsy(user.displayName) ||
+      this.isFalsy(user.mail)
     );
   };