Add check whether $cancellable is an object

Author: s-bronstein

src/CancellationQueue.php

@@ -19,7 +19,7 @@ public function __invoke()
 
     public function enqueue($cancellable)
     {
-        if (!\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
+        if (!\is_object($cancellable) || !\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
             return;
         }