After selecting the field, the operator editor should be activated, and then the value editor,
or the user should be able to do that with the tab key.

RadDataFilter should provide public API for BeginEdit passing a node and which part of it exactly to enter edit mode.

Use attached to reproduce. 


Workaround:
class MyDataFilter : RadDataFilter
{
    protected override RadTreeViewElement CreateTreeViewElement()
    {
        return new MyDataFilterElement();
    }
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadDataFilter).FullName;
        }
    }
}
class MyDataFilterElement : RadDataFilterElement
{
    protected override IVirtualizedElementProvider<RadTreeNode> CreateElementProvider()
    {
        return new MyProvider(this);
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadDataFilterElement);
        }
    }
}
class MyProvider : DataFilterElementProvider
{
    public MyProvider(RadTreeViewElement treeViewElement) : base(treeViewElement)
    { }
    public override IVirtualizedElement<RadTreeNode> CreateElement(RadTreeNode data, object context)
    {
        try
        {
            return base.CreateElement(data, context);
        }
        catch
        {
            RadMessageBox.Show("Invalid expression");
        }
        return null;
    }
   
}