Custom Editor creation

[stefnotch]

Issue description:

I have been trying to create a custom editor for an object and ran into an issue.
A NullReferenceException is being thrown by this method:

FlaxAPI/FlaxEditor/CustomEditors/CustomEditorsUtil.cs

Line 76 in 36aa542

 internal static CustomEditor CreateEditor(Type targetType, bool canUseRefPicker = true) 

Steps to reproduce:

Editor/MyScriptEditor.cs

    public class Hi
    {
        public string test;
    }

    [CustomEditor(typeof(MyScript))]
    public class MyScriptEditor : GenericEditor
    {
        public override void Initialize(LayoutElementsContainer layout)
        {
            layout.Label("My Custom Editor", TextAlignment.Center);
            var group = layout.Group("Inner group");
            try
            {
                //Edit: As noted below, this should be a ReadOnlyValueContainer or CustomValueContainer 
                var n = new ValueContainer(typeof(Hi)) { new Hi() }; 
                layout.Object(n); // NullReferenceException
            }
            catch (Exception e)
            {
                Debug.LogError(e);
            }
            base.Initialize(group);
        }
    }

Minimal reproduction project:

Flax version:

0.4.6181

[stefnotch]

For future reference, an alternative is using a custom editor manually with proxy object like here

var x = new CustomEditorPresenter(null);
x.Panel.Parent = layout.ContainerControl;
x.Select(new Hi());

[Wojciech Figat]

You used the wrong values container implementation. For custom values access use ReadOnlyValueContainer or CustomValueContainer which gives even more flexibility.