handle exception and set dtype value to JSON, when column type JSON (#5644)

* handle exception and set dtype value to JSON, when column type JSON

* review comment

* only when we handle exception

superset/views/core.py

@@ -2252,6 +2252,8 @@ class Superset(BaseSupersetView):
             try:
                 dtype = '{}'.format(col['type'])
             except Exception:
+                # sqla.types.JSON __str__ has a bug, so using __class__.
+                dtype = col['type'].__class__.__name__
                 pass
             payload_columns.append({
                 'name': col['name'],