Sharepoint 2010 designer workflow automatic triggering error

I am creating a Review and Approval workflow on a sharepoint list item through designer.
So i wanted to automatically trigger approval workflow when review workflow completes, but when i do so one of the custom action in the approval workflow is throwing a null reference exception, but if i manually start the second workflow that is approval workflow it is working fine.

what could be the issue here.Will it make any previlage issue when workflows start automatically.

Thanks in advance .