Fixed code break.

framework/web/User.php

@@ -194,6 +194,8 @@ class User extends Component
         if ($identity instanceof IdentityInterface) {
             $this->_identity = $identity;
             $this->_access = [];
+        } elseif ($identity === null) {
+            $this->_identity = null;
         } else {
             throw new InvalidValueException('The identity object must implement IdentityInterface.');
         }