fix: handler null after variable collision

src/Prime/Notification/NotificationBuilder.php

@@ -539,8 +539,9 @@ class NotificationBuilder implements NotificationBuilderInterface
      */
     public function handler($handler)
     {
-        $handler = $this->envelope->get('Flasher\Prime\Stamp\HandlerStamp');
-        if ($handler instanceof HandlerStamp) {
+        $stamp = $this->envelope->get('Flasher\Prime\Stamp\HandlerStamp');
+
+        if ($stamp instanceof HandlerStamp) {
             throw new \LogicException('You cannot change the handler of a notification once it has been set.');
         }