For some of the items it worked very well, but why is it showing me errors now?
Please check mentioned InvokeWorkflow Files and their Argument if all have the proper values. This error means that variable/argument don’t have any reference (is not assigned anywhere).
In the flow, check the right side expression of the out_UploadID.
One of the variables are not set.
Check if you are getting a value in it by using a write line/ check if the get of the upload id is returning a value.