you need to first identify at which moment the error occurs. You can place a Rethrow in the Catch of the INIT state, which may help. If not, place many message boxes or Debug in steps, until the exception in the INIT state is thrown. Then, this will tell at which point it occurs.
I suspect it’s on one of your Invokes, though. Make sure your IN and OUT directions on the invoke matches the workflow being invoked. But, I think it is because something is coming in empty and the workflow can’t get invoked. You can check each variable by using
(variable is empty).tostring in a message box or write line.
If you identify “where” the error occurs, and you still having trouble figuring out what is causing, feel free to post image of some of the code in that place and the arguments inside the workflow being invoked.