Update nested Json with generic lists - System.ArgumentNullException: 'Value cannot be null. Arg_ParamName_Name'

Hello.

I´m trying to update a Json-Column with nested complex objects. I can create the inital object without a hassle.
When I try to update (add a new object to a list inside the Json-Field) I get an exception:

```
System.ArgumentNullException: 'Value cannot be null. Arg_ParamName_Name'

at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument) in System\ThrowHelper.cs:line 247
at System.Collections.Generic.Dictionary`2.FindValue(TKey key) in System.Collections.Generic\Dictionary.cs:line 1036
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value) in System.Collections.Generic\Dictionary.cs:line 1388
at Microsoft.EntityFrameworkCore.Update.ModificationCommand.g__HandleJson|41_4(List`1 columnModifications, <>c__DisplayClass41_0& ) in Microsoft.EntityFrameworkCore.Update\ModificationCommand.cs:line 517
at Microsoft.EntityFrameworkCore.Update.ModificationCommand.GenerateColumnModifications() in Microsoft.EntityFrameworkCore.Update\ModificationCommand.cs:line 302
at Microsoft.EntityFrameworkCore.Update.ModificationCommand.<>c.b__33_0(ModificationCommand command) in Microsoft.EntityFrameworkCore.Update\ModificationCommand.cs:line 146
at Microsoft.EntityFrameworkCore.Internal.NonCapturingLazyInitializer.EnsureInitialized[TParam,TValue](TValue& target, TParam param, Func`2 valueFactory) in Microsoft.EntityFrameworkCore.Internal\NonCapturingLazyInitializer.cs:line 16
at Microsoft.EntityFrameworkCore.Update.ModificationCommand.get_ColumnModifications() in Microsoft.EntityFrameworkCore.Update\ModificationCommand.cs:line 146
at Microsoft.EntityFrameworkCore.Update.Internal.CommandBatchPreparer.d__10.MoveNext() in Microsoft.EntityFrameworkCore.Update.Internal\CommandBatchPreparer.cs:line 80
at Microsoft.EntityFrameworkCore.Update.Internal.CommandBatchPreparer.d__8.MoveNext() in Microsoft.EntityFrameworkCore.Update.Internal\CommandBatchPreparer.cs:line 63
at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.d__9.MoveNext() in Microsoft.EntityFrameworkCore.Update.Internal\BatchExecutor.cs:line 111
at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.d__111.MoveNext() in Microsoft.EntityFrameworkCore.ChangeTracking.Internal\StateManager.cs:line 851
at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.d__115.MoveNext() in Microsoft.EntityFrameworkCore.ChangeTracking.Internal\StateManager.cs:line 927
at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.d__7`2.MoveNext()
at Microsoft.EntityFrameworkCore.DbContext.d__63.MoveNext() in Microsoft.EntityFrameworkCore\DbContext.cs:line 351
at Microsoft.EntityFrameworkCore.DbContext.d__63.MoveNext() in Microsoft.EntityFrameworkCore\DbContext.cs:line 375
at EfTestJson.Data.ApplicationDbContext.d__7.MoveNext() in D:\Projects\EFCore\EfTestJson\EfTestJson\Data\ApplicationDbContext.cs:line 31
at Program.<$>d__0.MoveNext() in D:\Projects\EFCore\EfTestJson\EfTestJson\Program.cs:line 86
at Program.(String[] args)
```
I´m using the .ToJson function. The configuration looks like this:

```
internal sealed class PollConfiguration : IEntityTypeConfiguration
{
public void Configure(EntityTypeBuilder builder)
{
builder.ToTable("Polls");

builder.HasKey(c => c.Id);
builder.Property(c => c.Title).HasMaxLength(255).IsRequired();
builder.Property(c => c.Description).HasMaxLength(1000).IsRequired(false);
builder.Property(c => c.Start).IsRequired(false);
builder.Property(c => c.End).IsRequired(false);
builder.Property(c => c.Status).IsRequired(true);
builder.OwnsMany(c => c.Categories, d =>
{
d.ToJson();
d.OwnsMany(e => e.Tasks, e =>
{
e.ToJson();
e.OwnsMany(f => f.TasksUsers, g =>
{
g.ToJson();
g.Property(f => f.Percent).HasPrecision(18, 2);
});
});
});
builder.Property(c => c.CreationDate).IsRequired(false);
builder.Property(c => c.LastUpdate).IsRequired(false);
builder.Property(c => c.Version).IsRowVersion();
}
}
```
So there is a poll object. This contains a list of "categories" stored as Json. Each category has a list of tasks. The tasks have a list of "TaskUsers".

The workflow is like this:
- the user creates a poll (works)
- later the user adds a category (works - the category does not contain tasks at this time)
- then the user adds a task to a category - this is when the exception occurs

You can find a repo here: [https://github.com/FM1973/EfTestJson.git](https://github.com/FM1973/EfTestJson.git)

What am I missing?
Any help is appreciated. Thanks!

PS: when I add a category already containing tasks there is no error. When I try to add another task later, the error occurs again.

Provider and version information

EF Core version: 8.0.4
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 8
Operating system: Windows 10
IDE: Visual Studio 2022 17.9.6