Update cfdm/read_write/netcdf/netcdfread.py

Co-authored-by: David Hassell <davidhassell@users.noreply.github.com>

Author: bnlawrence

cfdm/read_write/netcdf/netcdfread.py

@@ -544,7 +544,7 @@ def dataset_open(self, dataset, flatten=True, verbose=None):
 
         g["cdl_filename"] = cdl_filename
 
-        filesystem = g.get("filesystem")
+        filesystem = g["filesystem"]
         if filesystem is not None:
             # --------------------------------------------------------
             # A pre-authenticated filesystem was provided: open the