SelectIndex=-1 when Item's value=empty if AllowCustomText=True

[tomexou]

RadCombox version is 2017 R2 (2017.05.03).

When user chooses second item: "empty", the SelectedIndex=1 when AllowCustomText=False,
but SelectedIndex=-1 if the value is empty when AllowCustomText=True.

I think it's a bug for empty value in AllowCustomText mode.

<telerik:RadComboBox ID="Type" runat="server"
    AllowCustomText="true"
    RenderMode="Lightweight"
    CollapseAnimation-Duration="0"
    ExpandAnimation-Duration="0">
    <Items>
        <telerik:RadComboBoxItem Text="first" Value="1" />
        <telerik:RadComboBoxItem Text="empty" Value="" />
        <telerik:RadComboBoxItem Text="third" Value="2" />
    </Items>
</telerik:RadComboBox>