If the AllowCustom parameter is set to true and the OnChange fires during the data load, the value in the OnChange handler is null. Press Enter before the data load ends and the OnChange handler will give null instead of the current input value.
If I enter a text and press enter when there are no items (yet), I expect it to call the OnChange with that text. Even with a static list of zero items (replace 'Read=' with 'Data=' en provide a new List<>()), the OnChange is passed a null value.
If the OnChange fires before the empty dropdown get opened, it passes a null value, if it fires after the empty dropdown opens, it passes the entered text. That doesn't sound logical to me. Why needs the (empty) dropdown to be open before the data gets passed? And if that is necessary, why gets the OnChange fired before the dropdown is opened, and doesn't wait till it is open and then fire with the entered data?