Invoke SAP_EXECUTION workflow: Object reference not set to an instance of an

I have given correct arguments even though it is throwing error.

what might be reason?

It would be easier to troubleshoot by going into your SAP_EXECUTION workflow and add logs before activities that you suspect would be the culprit (most of the time it is assign activities). In the logs include the variables you are using to make sure that the variable really have a content during execution.