v10: Exception when trying to view locked pages with no logged in user

[KevinJump]

When you try and load a site that is locked - without being logged in you get an error:

 An unhandled exception has occurred while executing the request.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Our.Umbraco.MaintenanceMode.BackofficeUserAccessor.get_BackofficeUser()
   at Our.Umbraco.MaintenanceMode.Middleware.MaintenanceRedirectMiddleware.InvokeAsync(HttpContext context, IBackofficeUserAccessor backofficeUserAccessor)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

this seems to be an internal to umbraco thing.