34058: Fix UI Grid error when DAG has been removed. (#36028)

Author: avkirilishin

airflow/www/views.py

@@ -3013,6 +3013,9 @@ def legacy_graph(self):
     def graph(self, dag_id: str, session: Session = NEW_SESSION):
         """Redirect to the replacement - grid + graph. Kept for backwards compatibility."""
         dag = get_airflow_app().dag_bag.get_dag(dag_id, session=session)
+        if not dag:
+            flash(f'DAG "{dag_id}" seems to be missing from DagBag.', "error")
+            return redirect(url_for("Airflow.index"))
         dt_nr_dr_data = get_date_time_num_runs_dag_runs_form_data(request, session, dag)
         dttm = dt_nr_dr_data["dttm"]
         dag_run = dag.get_dagrun(execution_date=dttm)