The error you are getting is because you have not set some argument value in the invoke or inside the get transaction data process.
I guess one of the inputs to the workflow is null.
Please debug and check if any of the required input is null.
For example, either TransactionData or WIList is null.
You’re right! My TransactionData forgot to set to a global variable, causing my GetTransactionData to get no value. Thank you very much for your help! (≧ڡ≦*)