Add check whether $cancellable is an object

Author: s-bronstein

src/Internal/CancellationQueue.php

@@ -22,7 +22,7 @@ public function __invoke(): void
 
     public function enqueue($cancellable): void
     {
-        if (!\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
+        if (!\is_object($cancellable) || !\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
             return;
         }