Fix T43900: "Operator Mesh Add" template fails

Another script that was missing the lookup_table call.

release/scripts/templates_py/operator_mesh_add.py

@@ -90,6 +90,7 @@ class AddBox(bpy.types.Operator):
             bm.verts.new(v_co)
 
         for f_idx in faces:
+            bm.verts.ensure_lookup_table()
             bm.faces.new([bm.verts[i] for i in f_idx])
 
         bm.to_mesh(mesh)