Not always happening, but I will try to give a good way to reproduce :

1. In web project, go to the CMS, click to open the NotificatorTool, close it, reopen it, etc..
2. Make sure the tool is closed
3. Add a page
4. Getting the error :

5. TypeError: Cannot read property 'add' of null
   TypeError: Cannot read property 'add' of null
   at constructor._onNotificationAdded (http://localhost:39006/cms/plugins/core-ui/resources/js/Ametys/plugins/coreui/notification/NotificationTool.js:316:28)
   at constructor.<anonymous> (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:6696:35)
   at constructor.fire (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:20326:32)
   at constructor.doFireEvent (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:21244:47)
   at constructor.prototype.doFireEvent (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:55078:35)
   at constructor.fireEventArgs (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:21097:26)
   at constructor.fireEvent (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:21056:25)
   at constructor.onCollectionAddItems (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:90002:20)
   at constructor.onCollectionAdd (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:89948:14)
   at constructor.notify (http://localhost:39006/cms/plugins/extjs6/resources/ext-all-debug.js:72923:28)