fix: Fixing schemas_allowed_for_upload field in database connection UX (#15750)

superset/models/core.py

@@ -20,6 +20,7 @@ import enum
 import json
 import logging
 import textwrap
+from ast import literal_eval
 from contextlib import closing
 from copy import deepcopy
 from datetime import datetime
@@ -668,6 +669,10 @@ class Database(
         self,
     ) -> List[str]:
         allowed_databases = self.get_extra().get("schemas_allowed_for_csv_upload", [])
+
+        if isinstance(allowed_databases, str):
+            allowed_databases = literal_eval(allowed_databases)
+
         if hasattr(g, "user"):
             extra_allowed_databases = config["ALLOWED_USER_CSV_SCHEMA_FUNC"](
                 self, g.user