Exception when marking all notifications as read

Description

Steps to reproduce

1. Use a site that is configured to use the multiplexing role and membership providers. For example: https://github.com/episerver/alloy-webforms-template.
2. Configure the Sql provider as provider1, as shown in the example below.
3. Create a SQL user that starts with the same letters as a windows user. Example: Window user is "kim" so we create a sql user called "kimi".
4. Create a project.
5. In the project overview, go to project comments.
6. Create two comments where you mention one user in each: "kim" and "kimi".
7. As the window user "kim", open the notification area and press the "Mark all as read" button.

Expected: To mark all notifications as read

Actual: 500 error from the server. Exception details posted below.

Note: If I change the order of the Sql and Window providers in the multiplexing provider, this issue no longer happens.

Example provider configuration:

    <roleManager enabled="true" defaultProvider="MultiplexingRoleProvider" cacheRolesInCookie="true">
      <providers>
        <clear />
        <add name="MultiplexingRoleProvider" type="EPiServer.Security.MultiplexingRoleProvider, EPiServer.Framework.AspNet" provider1="SqlServerRoleProvider" provider2="WindowsRoleProvider" providerMap1="SqlServerMembershipProvider" providerMap2="WindowsMembershipProvider" />
        <add name="WindowsRoleProvider" applicationName="EPiServerSample" type="EPiServer.Security.WindowsRoleProvider, EPiServer.Cms.AspNet" />
        <add name="SqlServerRoleProvider" connectionStringName="EPiServerDB" applicationName="EPiServerSample" type="System.Web.Security.SqlRoleProvider, System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
      </providers>
    </roleManager>
    <membership defaultProvider="MultiplexingMembershipProvider" userIsOnlineTimeWindow="10" hashAlgorithmType="HMACSHA512">
      <providers>
        <clear />
        <add name="MultiplexingMembershipProvider" type="EPiServer.Security.MultiplexingMembershipProvider, EPiServer.Framework.AspNet" provider1="SqlServerMembershipProvider" provider2="WindowsMembershipProvider" />
        <add name="WindowsMembershipProvider" type="EPiServer.Security.WindowsMembershipProvider, EPiServer.Cms.AspNet" deletePrefix="BUILTIN\" searchByEmail="true" />
        <add name="SqlServerMembershipProvider" type="System.Web.Security.SqlMembershipProvider, System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" connectionStringName="EPiServerDB" requiresQuestionAndAnswer="false" applicationName="EPiServerSample" requiresUniqueEmail="true" passwordFormat="Hashed" maxInvalidPasswordAttempts="5" minRequiredPasswordLength="3" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10" passwordStrengthRegularExpression="" />
      </providers>
    </membership>

Exception details:

[NullReferenceException: Object reference not set to an instance of an object.]
   EPiServer.Notification.Internal.<MarkUserNotificationsAsReadAsync>d__47.MoveNext() +65
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +99
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +61
   EPiServer.Notification.Internal.<MarkUserNotificationsAsReadAsync>d__46.MoveNext() +211
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +99
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +61
   EPiServer.Cms.Shell.UI.Rest.Notifications.<MarkAllAsReadAsync>d__6.MoveNext() +415
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +99
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +61
   EPiServer.Cms.Shell.UI.Rest.Notifications.<MarkAllAsRead>d__5.MoveNext() +165
   System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +99
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +61
   System.Web.Mvc.Async.TaskAsyncActionDescriptor.EndExecute(IAsyncResult asyncResult) +97
   System.Web.Mvc.Async.<>c__DisplayClass37.<BeginInvokeAsynchronousActionMethod>b__36(IAsyncResult asyncResult) +17
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +49
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +226
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +226
   System.Web.Mvc.Async.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +26
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +100
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27
   EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult) +38
   EPiServer.Shell.Services.Rest.RestHttpHandler.EndProcessRequest(IAsyncResult result) +28
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9748665
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +48
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +159