Fix some json and cadnano tests

scadnano/scadnano.py

@@ -5668,6 +5668,9 @@ def to_cadnano_v2_serializable(self, name: str = '') -> Dict[str, Any]:
         '''
         for strand in self.strands:
             for domain in strand.domains:
+                if isinstance(domain, Extension):
+                    raise ValueError(
+                        'We cannot handle designs with Extensions as it is not a cadnano v2 concept')
                 if isinstance(domain, Loopout):
                     raise ValueError(
                         'We cannot handle designs with Loopouts as it is not a cadnano v2 concept')

tests/scadnano_tests.py

@@ -1576,9 +1576,9 @@ def test_extension(self) -> None:
         """
         design: sc.Design = sc.Design(helices=[sc.Helix(max_offset=100)], grid=Grid.square)
         sb = design.draw_strand(0, 0)
-
-        sb.extension_3p(5)
         sb.to(10)
+        sb.as_scaffold()
+        sb.extension_3p(5)
 
         with self.assertRaises(ValueError) as context:
             design.to_cadnano_v2_json()
@@ -1671,7 +1671,7 @@ def test_from_json__three_strands(self) -> None:
         scaf_loop = scaf.domains[2]
         scaf_ss2 = scaf.domains[3]
 
-        self.assertEqual(3, scaf_loop.num_bases)
+        self.assertEqual(3, scaf_loop.length)
 
         self.assertEqual(1, st_l_ss0.helix)
         self.assertEqual(0, st_l_ss1.helix)