souregrid加入combobox问题。高分求解

C# code

            SourceGrid.Cells.Editors.ComboBox externalIdEditor = new SourceGrid.Cells.Editors.ComboBox(typeof(int));            externalIdEditor.StandardValues = new int[] { 1, 2, 3, 4 };            externalIdEditor.StandardValuesExclusive = true;            externalIdEditor.Control.FormattingEnabled = true;            externalIdEditor.AllowNull = false;                                    DevAge.ComponentModel.Validator.ValueMapping mapping = new DevAge.ComponentModel.Validator.ValueMapping();            mapping.ValueList = new int[] { 1, 2, 3, 4 };            mapping.DisplayStringList = new string[] { "Reference 1", "Reference 2", "Reference 3", "Reference 4" };            mapping.BindValidator(externalIdEditor);

例子29加入了combobox，为什么
if (mView.AllowNew == false)
{
mView.AllowNew = true;
mView.AddNew();
mView.AllowNew = false;
}

新增一行的时候，combobox列显示错误？那个cell单元显示Error:Cannot convert <null> to display String.

请问有没人知道啊

[解决办法]
Cannot convert <null> to display String

没有设置值