Hi,

I am facing this issue when migrating from .NET Core 2.2 to .NET Core 3.1:

Steps to reproduce

public class Dashboard
    {
        public Dashboard()
        {
            Widgets = new HashSet<Widget>();
            Layouts = new List<Layout>();
        }
        public string Name { get; set; }
        public List<Layout> Layouts { get; set; }
        public virtual ICollection<Widget> Widgets { get; set; }
    }

public class Layout
    {
        public int Width { get; set; }
        public int Height { get; set; }
       // removed other feilds for clarity 
    }

//Entity Configuration
builder.Property(d => d.Name)
                .IsRequired();
            builder.Property(w => w.Layouts).HasConversion(
                v => JsonConvert.SerializeObject(v,
                    new JsonSerializerSettings {NullValueHandling = NullValueHandling.Ignore}),
                v => JsonConvert.DeserializeObject<List<Layout>>(v,
                    new JsonSerializerSettings {NullValueHandling = NullValueHandling.Ignore}));

Exception is thrown when following query is executed:

var result = await _context.Dashboards.AsNoTracking().Select(d => new DashboardModel
                {
                    Id = d.Id,
                    Name = d.Name,
                    Layouts = d.Layouts.Select(l => new LayoutDto {H = l.Height, W = l.Width}).ToList()
                }).ToListAsync(cancellationToken);

It works fine if don't use Select or Select without Layouts.

Got the following exception:

System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.AddCollectionProjection(ShapedQueryExpression shapedQueryExpression, INavigation navigation, Type elementType)
at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMemberAssignment(MemberAssignment memberAssignment)
at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMemberInit(MemberInitExpression memberInitExpression)
at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
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__DisplayClass12_01.<ExecuteAsync>b__0() at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func1 compiler)
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func1 compiler) at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable1.GetAsyncEnumerator(CancellationToken cancellationToken)
at System.Runtime.CompilerServices.ConfiguredCancelableAsyncEnumerable1.GetAsyncEnumerator() at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable1 source, CancellationToken cancellationToken)

Further technical details

EF Core version: 3.1.5
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET Core 3.1
Operating system: Windows
IDE: Visual Studio 2019 16.6.3