RadPropertyGrid calls the TypeConverter methods GetStandardValuesSupported and GetStandardValues passing null as parameter when it should pass the property item.
Code to reproduce:

this.radPropertyGrid1.SelectedObject = new SomeClass();

public class SomeClass
{
    [TypeConverter(typeof(CustomValueConverter))]
    public string CustomValue
    {
        get;
        set;
    }
}

public class CustomValueConverter : TypeConverter    
{
    public override bool GetStandardValuesSupported(ITypeDescriptorContext context)
    {
        return true;
    }

    public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
    {
        return base.GetStandardValues(context);
    }
}