EF Core left join with DTO null reference exception

I have two methods, each of them has one or two inner joins and returns IQueryable.

When I want to join those two methods using leftJoin:

var project = from p in _projectManager.GetList()
              join c in _organizationManager.GetList() on p.CustomerId equals c.Id into tmpCustomer
              from c2 in tmpCustomer.DefaultIfEmpty()
              select new
              {
                  Customer = p.CustomerId.HasValue ? "1": string.Empty
              }

I get the error:

HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=Microsoft.EntityFrameworkCore.Relational
StackTrace:
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.ProjectionShaper.TypedProjectionShaper`3.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.CompositeShaper.TypedCompositeShaper`5.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.CompositeShaper.TypedCompositeShaper`5.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.ProjectionShaper.TypedProjectionShaper`3.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.BufferlessMoveNext(DbContext _, Boolean buffer)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.MoveNext()
   at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ResultEnumerable`1.GetEnumerator()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass15_1`1.<CompileQueryCore>b__0(QueryContext qc)
   at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source)
   at StandardWT.Core.Services.ProjectService.GetAttributes(Int64 projectId) in c:\SourceCode\WorkTimeIntegrationService\StandardWT.Core\Services\ProjectService.cs:line 59
   at StandardWT.Web.Controllers.Api.ProjectAttributeValueController.Get(Int64 projectId) in c:\SourceCode\WorkTimeIntegrationService\StandardWT.Web\Controllers\Api\ProjectAttributeValueController.cs:line 27
   at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)
   at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeActionMethodAsync>d__12.MoveNext()