Efcore: Object reference not set to an instance of an object exception when using NET Topology Suite in Select projection

[appimpact]

After the upgrade from .NET Core 3.1 to .NET 5.0, we have noticed a systematic error on all our queries that include NET Topology Suite but only within certain scenarios when there is a nested Select statement.

Those queries are now throwing 'Object reference not set to an instance of an object' exception.

Full example can be found here:
https://github.com/App-Impact-Labs/ISSUE-EFNetTopologySuiteProjectionQueryError

In the example above, we have three entities:
```C#
public class MedicalService
{
public int Id { get; set; }

    public string Name { get; set; }

    public virtual IList<HealthInstitutionMedicalServiceLink> HealthInstitutionLinks { get; set; }
}

```C#
    public class HealthInstitution
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public Point Coordinates { get; set; }

        public virtual IList<HealthInstitutionMedicalServiceLink> MedicalServiceLinks { get; set; }
    }

```C#
public class HealthInstitutionMedicalServiceLink
{
public int Id { get; set; }

    public int HealthInstitutionId { get; set; }
    public virtual HealthInstitution HealthInstitution { get; set; }

    public int MedicalServiceId { get; set; }
    public virtual MedicalService MedicalService { get; set; }
}

1) This query works (coordinates are commented out):

```C#
            HealthInstitutionGetDetailsRes dto = dbContext.HealthInstitutions
                .Select(hi => new HealthInstitutionGetDetailsRes
                {
                    Id = hi.Id,
                    Name = hi.Name,
                        //Coordinates = new HealthInstitutionGetDetailsRes.GeolocationDto
                        //{
                        //    Latitude = hi.Coordinates.Y,
                        //    Longitude = hi.Coordinates.X
                        //},
                        MedicalServiceIds = hi.MedicalServiceLinks.Select(msl => msl.MedicalServiceId)
                })
                .FirstOrDefault();

2) This one works also (ids are commented out):

```C#
HealthInstitutionGetDetailsRes dto = dbContext.HealthInstitutions
.Select(hi => new HealthInstitutionGetDetailsRes
{
Id = hi.Id,
Name = hi.Name,
Coordinates = new HealthInstitutionGetDetailsRes.GeolocationDto
{
Latitude = hi.Coordinates.Y,
Longitude = hi.Coordinates.X
},
//MedicalServiceIds = hi.MedicalServiceLinks.Select(msl => msl.MedicalServiceId)
})
.FirstOrDefault();

3) However full query with both coordinates and ids is not working (throws an exception):

```C#
            HealthInstitutionGetDetailsRes dto = dbContext.HealthInstitutions
                .Select(hi => new HealthInstitutionGetDetailsRes
                {
                    Id = hi.Id,
                    Name = hi.Name,
                    Coordinates = new HealthInstitutionGetDetailsRes.GeolocationDto
                    {
                        Latitude = hi.Coordinates.Y,
                        Longitude = hi.Coordinates.X
                    },
                    MedicalServiceIds = hi.MedicalServiceLinks.Select(msl => msl.MedicalServiceId)
                })
                .FirstOrDefault();

Include stack traces

   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SqlFunctionExpression.GetHashCode()
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.PushdownIntoSubquery()
   at Microsoft.EntityFrameworkCore.Query.Internal.CollectionJoinApplyingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
   at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
   at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.Internal.CollectionJoinApplyingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPostprocessor.Process(Expression query)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source)

Include provider and version information

EF Core version: 5.0.0
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 5.0
Operating system: Windows 10 Enterprise
IDE: Visual Studio 2019 16.8.0

[maumar]

I verified this reproduces with 5.0 bits but works correctly on current main. Generated sql:

SELECT [t].[Id], [t].[Name], [t].[c], [t].[c0], [h0].[MedicalServiceId], [h0].[Id]
FROM (
    SELECT TOP(1) [h].[Id], [h].[Name], [h].[Coordinates].Lat AS [c], [h].[Coordinates].Long AS [c0]
    FROM [HealthInstitutions] AS [h]
) AS [t]
LEFT JOIN [HealthInstitutionMedicalServiceLinks] AS [h0] ON [t].[Id] = [h0].[HealthInstitutionId]
ORDER BY [t].[Id], [h0].[Id]

[smitpatel]

This https://github.com/dotnet/efcore/commit/43a5493251faf2591eb2f50abef4b8597374642f#diff-a307bbe9617539d71c4d3d72606c8b7328be03419f04e034b1e492977ca82aa3 fixed it. But we can extract out fix here for patch if needed. It is missing a null check in hash computation.

[appimpact]

We have to do query splitting to fix our systems, so if it is possible to embed this for the next patch it would be appreciated.