NRE in GroupJoin to parent when using query types

[sandersaares]

I am getting NullReferenceException from GroupJoin.

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method(Closure , MaterializationContext )
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.UnbufferedEntityShaper`1.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.UnbufferedOffsetEntityShaper`1.Shape(QueryContext queryContext, ValueBuffer& valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider._GroupJoin[TOuter,TInner,TKey,TResult](RelationalQueryContext queryContext, IEnumerable`1 source, IShaper`1 outerShaper, IShaper`1 innerShaper, Func`2 innerKeySelector, Func`3 resultSelector)+MoveNext()
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at ConsoleApp13.Program.Main(String[] args) in C:\Source\ConsoleApp13\ConsoleApp13\Program.cs:line 28

This only happens if I am using query types. With entity types it works fine.

Possibly related: #9892

Steps to reproduce

Use migrations to generate the database and seed data.

using Microsoft.EntityFrameworkCore;
using System;
using System.Linq;

namespace ConsoleApp13
{
    internal class Program
    {
        public static readonly Guid ParentWithChildId = new Guid("bcee42c7-ad08-4a43-b9ea-ecb62939666e");
        public static readonly Guid ParentWithoutChildId = new Guid("d65de666-7a3c-49bb-90e2-c780d10832aa");
        public static readonly Guid ChildId = new Guid("5d846ff2-9db8-4a45-a939-3746df161eb4");

        private static void Main(string[] args)
        {
            using (var dc = new DataContext())
            {
                // Works fine if the types are entity types.
                var joinedEntity = dc.Parent
                    .GroupJoin(dc.Child, p => p.Id, c => c.ParentId, (parent, children) => new { parent, children })
                    .ToArray();

                Console.WriteLine($"{joinedEntity.Length} rows.");

                foreach (var row in joinedEntity)
                    Console.WriteLine($"entity row {row.parent.Id} has {row.children?.Count()} children");

                // But if they are query types, fails with NRE.
                var joinedQuery = dc.ParentQuery
                    .GroupJoin(dc.ChildQuery, p => p.Id, c => c.ParentId, (parent, children) => new { parent, children })
                    .ToArray();

                Console.WriteLine($"{joinedQuery.Length} rows.");

                foreach (var row in joinedQuery)
                    Console.WriteLine($"query row {row.parent.Id} has {row.children?.Count()} children");
            }
        }
    }

    public class ParentRow
    {
        public Guid Id { get; set; }
    }

    public class ChildRow
    {
        public Guid Id { get; set; }
        public Guid ParentId { get; set; }
    }

    public class ParentQueryRow
    {
        public Guid Id { get; set; }
    }

    public class ChildQueryRow
    {
        public Guid Id { get; set; }
        public Guid ParentId { get; set; }
    }

    public class DataContext : DbContext
    {
        public DbSet<ParentRow> Parent { get; set; }
        public DbSet<ChildRow> Child { get; set; }

        public DbQuery<ParentQueryRow> ParentQuery { get; set; }
        public DbQuery<ChildQueryRow> ChildQuery { get; set; }

        public DataContext()
        {
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
                optionsBuilder.UseSqlServer("Server=(local);Database=EFCore_Issue_Repro;Trusted_Connection=True;");
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<ParentRow>()
                .HasData(
                    new ParentRow
                    {
                        Id = Program.ParentWithChildId
                    },
                    new ParentRow
                    {
                        Id = Program.ParentWithoutChildId
                    }
                    );

            modelBuilder.Entity<ChildRow>()
                .HasData(
                    new ChildRow
                    {
                        Id = Program.ChildId,
                        ParentId = Program.ParentWithChildId
                    }
                );

            modelBuilder.Query<ParentQueryRow>()
                .ToView("Parent");
            modelBuilder.Query<ChildQueryRow>()
                .ToView("Child");
        }
    }
}

Further technical details

EF Core version: 2.2.1
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10 1809
IDE: Visual Studio 2017 15.9.5

[smitpatel]

Unsupported query as per #17068