SignInAsync throws unhelpful exception when `Conventions.FindIdentityProperty` is used.

[jbuedel]

We've mapped our document Ids to DocumentID. When registering a new user I call SignInManager.SignInAsync<AppUser>(...) and get an ArgumentNullException.

Overriding AppUser.Id, like so, fixes it.

   public class AppUser : IdentityUser
   {
      public override string? Id => DocumentID;
      public string DocumentID { get; set; }
   }

The exception is:

dbug: Raven.Identity.UserStore[0]
      Creating email reservation for xjs@dkdk
fail: Microsoft.AspNetCore.Server.Kestrel[13]
      Connection id "0HMC82JFR5V3L", Request id "0HMC82JFR5V3L:00000001": An unhandled exception was thrown by the application.
System.ArgumentNullException: Value cannot be null. (Parameter 'value')
   at System.Security.Claims.Claim..ctor(String type, String value, String valueType, String issuer, String originalIssuer, ClaimsIdentity subject, String propertyKey, String propertyValue)
   at System.Security.Claims.Claim..ctor(String type, String value)
   at Microsoft.AspNetCore.Identity.UserClaimsPrincipalFactory`1.GenerateClaimsAsync(TUser user)
   at Microsoft.AspNetCore.Identity.UserClaimsPrincipalFactory`2.GenerateClaimsAsync(TUser user)
   at Microsoft.AspNetCore.Identity.UserClaimsPrincipalFactory`1.CreateAsync(TUser user)
   at Microsoft.AspNetCore.Identity.SignInManager`1.CreateUserPrincipalAsync(TUser user)
   at Microsoft.AspNetCore.Identity.SignInManager`1.SignInWithClaimsAsync(TUser user, AuthenticationProperties authenticationProperties, IEnumerable`1 additionalClaims)
   at AMS.Eclipse.Server.Controllers.ApiAuthenticationController.Register(RegisterParams registerParams) in C:\Users\jbuedel\Projects\EclipsePro-Design\src\AMS.Eclipse.Server\Controllers\ApiAuthenticationController.cs:line 114
   at lambda_method(Closure , Object )
   at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.TaskOfActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)
Connection id "0HMC82JFR5V3L", Request id "0HMC82JFR5V3L:00000001": An unhandled exception was thrown by the application. Value cannot be null. (Parameter 'value')

Our DocumentStore creation looks like this:

         var store = new DocumentStore
         {
            Conventions =
            {
               FindIdentityProperty = prop => prop.Name == "DocumentID",
               MaxNumberOfRequestsPerSession = 200
            },
            Urls = new[] { url },
            Database = databaseName
         };

My thought is maybe IdentityUser.Id should be removed and instead somehow resolve the document id property through the convention perhaps?