Fixed a bug where post reminder modal didn't close (#29615)

* Fixed  abug where post reminder modal didnt close

* Fixed a test

webapp/channels/src/components/post_reminder_custom_time_picker_modal/post_reminder_custom_time_picker_modal.tsx

@@ -32,7 +32,8 @@ function PostReminderCustomTimePicker({userId, timezone, onExited, postId, actio
 
     const handleConfirm = useCallback((dateTime: Moment) => {
         actions.addPostReminder(userId, postId, toUTCUnix(dateTime.toDate()));
-    }, [actions, postId, userId]);
+        onExited();
+    }, [actions, postId, userId, onExited]);
 
     return (
         <DateTimePickerModal