Use the debugger: The chances are that you spelt "FileUpload1" wrong and it can't find it in this line:
FileUpload fuCOA = (FileUpload)gvRow.FindControl("FileUpload1");
If it isn't that, then it's likely that there is no file available - but you need to use the debugger to find out exactly which part of your statement is null to be sure.
We can't do that for you - we can;t run your code!