After upgrading to 4.9, the back end stops working from time to time with this error when logging in:

[ArgumentNullException: Value cannot be null.
Parameter name: source]
   System.Linq.Enumerable.SingleOrDefault(IEnumerable`1 source, Func`2 predicate) +2535654
   umbraco.BusinessLogic.User.GetApplications() +273
   umbraco.cms.presentation._umbraco.Page_Load(Object sender, EventArgs e) +64
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +51
   System.Web.UI.Control.OnLoad(EventArgs e) +92
   umbraco.BasePages.BasePage.OnLoad(EventArgs e) +14
   System.Web.UI.Control.LoadRecursive() +54
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +772