fix structured dataset

Author: mjreno

flopy/discretization/structuredgrid.py

@@ -1781,6 +1781,8 @@ def _layered_mesh_dataset(self, ds, modeltime=None):
         ds["mesh_face_nodes"].attrs["_FillValue"] = FILLNA_INT32
         ds["mesh_face_nodes"].attrs["start_index"] = np.int32(1)
 
+        return ds
+
     def _structured_dataset(self, ds, modeltime=None):
         lenunits = {0: "m", 1: "ft", 2: "m", 3: "m"}
 
@@ -1838,6 +1840,8 @@ def _structured_dataset(self, ds, modeltime=None):
         ds["x"].attrs["long_name"] = "Easting"
         ds["x"].attrs["bounds"] = "x_bnds"
 
+        return ds
+
     def _set_structured_iverts(self):
         """
         Build a list of the vertices that define each model cell and the x, y