NullText behavior on UltraComboBox

I am using a UltraComboBox from Infragistics in a forms Task Manager screen

The UltraComboBox has its dropdown button hidden, and AutoCompletion is active so the user can type a couple of letters to retrieve an user name开发者_开发百科 to assign someone to a task.

However, the user has the possibility to let the task "Not assigned" by clicking the eraser icon, setting the internal value of the ComboBox to null :

private void SetNotAssigned_Click(object sender, EventArgs e)
{
    AssignedToComboBox.Value = null;
}

Instead of having my ComboBox filled with its NullText property ("Not assigned"), it is blank. Why ?

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I just found why.

I had to put AlwaysInEditMode to false.

It looks like Infragistics is using a hidden TextBox that appears during edition and disappears afterwards. I suspect this TextBox behavior is not allways the expected one. AlwaysInEditMode seems to make that hidden TextBox on all the time.