fix(sqllab): null database with backend persistence (#19548)

superset-frontend/src/SqlLab/actions/sqlLab.js

@@ -1393,10 +1393,21 @@ export function queryEditorSetFunctionNames(queryEditor, dbId) {
           functionNames: json.function_names,
         }),
       )
-      .catch(() =>
-        dispatch(
-          addDangerToast(t('An error occurred while fetching function names.')),
-        ),
-      );
+      .catch(err => {
+        if (err.status === 404) {
+          // for databases that have been deleted, just reset the function names
+          dispatch({
+            type: QUERY_EDITOR_SET_FUNCTION_NAMES,
+            queryEditor,
+            functionNames: [],
+          });
+        } else {
+          dispatch(
+            addDangerToast(
+              t('An error occurred while fetching function names.'),
+            ),
+          );
+        }
+      });
   };
 }

superset/models/sql_lab.py

@@ -118,7 +118,7 @@ class Query(Model, ExtraJSONMixin):
             "changedOn": self.changed_on,
             "changed_on": self.changed_on.isoformat(),
             "dbId": self.database_id,
-            "db": self.database.database_name,
+            "db": self.database.database_name if self.database else None,
             "endDttm": self.end_time,
             "errorMessage": self.error_message,
             "executedSql": self.executed_sql,