owtestlearners: Fix migration of old settings

Author: lanzagar

Orange/widgets/evaluate/owtestlearners.py

@@ -714,7 +714,7 @@ def send_report(self):
     @classmethod
     def migrate_settings(cls, settings_, version):
         if version < 2:
-            if not hasattr(settings_["context_settings"][0], "attributes"):
+            if not hasattr(settings_.get("context_settings", [None])[0], "attributes"):
                 settings_["context_settings"][0].attributes = {}
             if settings_["resampling"] > 0:
                 settings_["resampling"] += 1