Check whether you passed input to in_WIList argument or not.
It seems that you are passing a null value to GetTransactionData.xaml when invoking it. Make sure you are passing the required values in required format to all the needed arguments. Specially, the transaction number, TransactionData (if you have a data table only), and in_WList argument which you have specified as a input argument for the workflow. I think the problem is with the In_WIList. Make sure the data is properly passed from INIT state to Get Transaction Data and it is being passed to this workflow.
Let me know whether it helps.