[Console] fixed Command::setApplication() when the argument is null

src/Symfony/Component/Console/Command/Command.php

@@ -77,7 +77,11 @@ class Command
     public function setApplication(Application $application = null)
     {
         $this->application = $application;
-        $this->setHelperSet($application->getHelperSet());
+        if ($application) {
+            $this->setHelperSet($application->getHelperSet());
+        } else {
+            $this->helperSet = null;
+        }
     }
 
     /**