Avoid possible UnboundLocalError for end_event

The error has been observed to occur in the log. Not trying
to display further error messages to why it is None.

src/ert/cli/main.py

@@ -13,7 +13,7 @@
 from ert.cli.monitor import Monitor
 from ert.cli.workflow import execute_workflow
 from ert.config import ErtConfig, QueueSystem
-from ert.ensemble_evaluator import EvaluatorServerConfig
+from ert.ensemble_evaluator import EndEvent, EvaluatorServerConfig
 from ert.mode_definitions import (
     ENSEMBLE_EXPERIMENT_MODE,
     ENSEMBLE_SMOOTHER_MODE,
@@ -128,6 +128,7 @@ def run_cli(args: Namespace, plugin_manager: Optional[ErtPluginManager] = None)
             out = sys.stderr
         monitor = Monitor(out=out, color_always=args.color_always)
         thread.start()
+        end_event: Optional[EndEvent] = None
         try:
             end_event = monitor.monitor(
                 status_queue, ert_config.analysis_config.log_path
@@ -139,7 +140,7 @@ def run_cli(args: Namespace, plugin_manager: Optional[ErtPluginManager] = None)
     thread.join()
     storage.close()
 
-    if end_event.failed:
+    if end_event is not None and end_event.failed:
         # If monitor has not reported, give some info if the job failed
         msg = end_event.msg if args.disable_monitoring else ""
         raise ErtCliError(msg)