In one of my WPF projects, I have a UserControl as following:

<UserControl x:Class="Repository.UI.SetupModule.CalcRate.UserControl1"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                    xmlns:ig="http://schemas.infragistics.com/xaml"
                    mc:Ignorable="d"
                    d:DesignHeight="300" d:DesignWidth="300">

    <Grid>
        <ig:XamComboEditor/>
    </Grid>
</UserControl>

I got the error on the XamComboEditor : Object reference not set to an instance of an object.  

The StackTrace is:

at Infragistics.Controls.Editors.ComboEditorBase`2.OnApplyTemplate()
at System.Windows.FrameworkElement.ApplyTemplate()

For testing purpose, I create a brand new WPF project, and add the same UserControl to the project, this time, there's no error on the XamCombeEditor.

I guess there must something in my project that causes the error, but unable to find out what it is.

please help, thanks.