While publishing .bpm files to web I received an error message

Hi guys,
While publishing .bpm files to web I received an error message. It threw the error message: "Object reference not set to an instance of an object". I received the same error message while exporting the same file to Word or PDF. I do not know the reason behind the issue. Have you any idea? Please help me to get a remedy of this issue.
Thanks in advance.
