Set property to null instead of unsetting.

This avoid triggering Controller::__get() on subsequent access to ViewVarsTrait::$viewClass.

src/View/ViewVarsTrait.php

@@ -74,7 +74,7 @@ public function createView($viewClass = null)
         $builder = $this->viewBuilder();
         if ($viewClass === null && $builder->getClassName() === null) {
             $builder->setClassName($this->viewClass);
-            unset($this->viewClass);
+            $this->viewClass = null;
         }
         if ($viewClass) {
             $builder->setClassName($viewClass);