fix: avoid null argument exception

src/Form/AssignmentType.php

@@ -166,6 +166,10 @@ class AssignmentType extends AbstractType
                     ->find($destinationId)
                 ;
 
+                if (null === $destination) {
+                    return;
+                }
+
                 $this->addPickupField($destination, $form);
             })
         ;