I need to have a multi-column header with two CheckBox. If I write this code:

<ig:GroupField Key="clmDayMonday">
<Header Text="<%$ Resources:Language, lbl_DayMon %>" />
<Columns>
<ig:BoundCheckBoxField Width="14%" DataFieldName="MonMorShift" DataType="System.Boolean"
Key="clmMonMorShift" CssClass="textAlignCenter">
<CheckBox CheckedImageUrl="../../Images/bg_check.png" UncheckedImageUrl="../../Images/bg_uncheck.png"
PartialImageUrl="../../Images/bg_uncheck.png" />
<Header Text="<%$ Resources:Language, clm_MorShift %>" />
</ig:BoundCheckBoxField>
<ig:BoundCheckBoxField Width="14%" DataFieldName="MonAftShift" DataType="System.Boolean"
Key="clmMonAftShift" CssClass="textAlignCenter">
<CheckBox CheckedImageUrl="../../Images/bg_check.png" UncheckedImageUrl="../../Images/bg_uncheck.png"
PartialImageUrl="../../Images/bg_uncheck.png" />
<Header Text="<%$ Resources:Language, clm_AftShift %>" />
</ig:BoundCheckBoxField>
</Columns>
</ig:GroupField>

it doesn't work.

I receive the error message below:

System.NullReferenceException: Object reference not set to an instance of an object.
at Infragistics.Web.UI.Framework.AppStyling.StyleBot.ImageDirectoryResolved()
at Infragistics.Web.UI.Framework.AppStyling.StyleBot.ResolveImageUrl(String imageUrl, Boolean useCache)
at Infragistics.Web.UI.Framework.ImageObject.ResolveImageUrl(String url, String defaultUrl)
at Infragistics.Web.UI.Framework.GridCheckBox.GetRenderImageUrl(CheckBoxState state)
at Infragistics.Web.UI.GridControls.BoundCheckBoxField.CalculateImageUrl(CheckBoxState state)
at Infragistics.Web.UI.GridControls.GridRecordItem.GenerateAttributes(Hashtable attributes)
at Infragistics.Web.UI.GridControls.XPathDataNavigable.GenerateAttributes()
at Infragistics.Web.UI.GridControls.XPathDataNavigable.get_XPathAttributes()
at Infragistics.Web.UI.GridControls.XPathDataNavigable.get_HasAttributes()
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.OutputToStream(XPathDataNavigator xpdn, XmlTextWriter stream, HtmlTextWriter htmlWriter)
at Infragistics.Web.UI.GridControls.XPathDataNavigator.get_OuterXml()
at Infragistics.Web.UI.GridControls.GridRenderer.RenderContents(HtmlTextWriter writer)
at Infragistics.Web.UI.Framework.RunBot.HandleRenderContents(HtmlTextWriter writer, RendererBase renderer)
at Infragistics.Web.UI.Framework.Data.FlatDataBoundControl.RenderContents(HtmlTextWriter writer)
at Infragistics.Web.UI.Framework.Data.FlatDataBoundControl.Render(HtmlTextWriter writer)
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)
at System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output)
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer)
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
at System.Web.UI.Page.Render(HtmlTextWriter writer)
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Could you tell me what's the problem?

Waiting for your answer,
Best regards.