Fix: 'urlListFileName' variable is None on job error.

Signed-off-by: Cristiano Urban <cristiano.urban@inaf.it>

transfer_service/retrieve_executor.py

@@ -456,6 +456,9 @@ class RetrieveExecutor(TaskExecutor):
         self.numBlocks = 0
         self.procBlocks = 0
         self.totalSize = 0
+        
+        # If an error occurs, the variable is None: just ignore it (the URL list is not generated)
+        if self.urlListFileName is not None:
             urlListFilePath = os.path.join(self.resDir, self.urlListFileName)
             try:
                 os.remove(urlListFilePath)