Error 500 when closing notification panel

Description

Was not able to reproduce locally BUT that may be due to using AD. Customer was able to reproduce on video easily and the database was in the error status when restored. Tested in Edge and Chrome.

1. User in group set for Approver in workflow
2. As user, make change to to a page that has approval workflow
3. Review and approve the changes for that page.
4. Once the page refreshed in the Experience Editor, there were 252 (92 in latest test) notifications in the notifications section. It apparently sent that many notifications to the same user.
5. Open the notification panel using the notification-bell, and then close it
   Observed:
   Server error 500

System.ArgumentNullException: Value cannot be null.
Parameter name: user
at EPiServer.Notification.Internal.DefaultNotifier.<MarkUserNotificationsAsReadAsync>d__47.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at EPiServer.Notification.Internal.DefaultNotifier.<MarkUserNotificationsAsReadAsync>d__46.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at EPiServer.Cms.Shell.UI.Rest.Notifications.NotificationService.<MarkAllAsReadAsync>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at EPiServer.Cms.Shell.UI.Rest.Notifications.NotificationStore.<MarkAllAsRead>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Mvc.Async.TaskAsyncActionDescriptor.EndExecute(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass8_0.<BeginInvokeAsynchronousActionMethod>b__1(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__11_0()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass11_1.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass11_1.<InvokeActionMethodFilterAsynchronouslyRecursive>b__2()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass3_6.<BeginInvokeAction>b__3()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass3_1.<BeginInvokeAction>b__5(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult)
at EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult)
at System.Web.HttpApplication.CallHandlerExecutionStep.<>c__DisplayClass7_0.<InvokeEndHandler>b__0()
at System.Web.HttpApplication.CallHandlerExecutionStep.InvokeEndHandler(IAsyncResult ar)
at System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)

Expected: No error should be exposed