c# - How to debug binding in WPF

I have a VS2008, C# WPF, Excel AddIn; Under some case, My addin throws exception like

A first chance exception of type 'System.InvalidOperationException' occurred in PresentationFramework.dll
A first chance exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll

but I could not locate where the exception comes from. I know it is b/c data binding. but unable to find out where. Each time I step in, VS tracks to a method which executes w/o error, then after that, the exception throws and but no clue of which line of code.

I have been struggling this for days and could not make a little progress.
Pls help out. thanks

Edit, it is too long to fit in comment. So I just put xaml file here. @xmal file that throws the exception. It is DataGridComboBoxColumn that throws the exception

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:dg="http://schemas.microsoft.com/wpf/2008/toolkit" >
    
        
        

        
        
                
    
    
        
            
        


        
            
                
                
                
            

            
                


                    
                        
                            
                        
                        
                            
                                
                            
                        

                        
                            
                                
                            
                        
                                                 
                
            

            
                  AutoGenerateColumns="False" 
                  Style="{StaticResource DataGridStyle}"
                  SelectionMode="Extended"
                  GridLinesVisibility="None"
                  HeadersVisibility="Column" RowDetailsVisibilityMode="VisibleWhenSelected"
                  ItemsSource="{Binding BasketItems, Mode=OneWay}" CanUserAddRows="False" CanUserDeleteRows="False"
                  SelectionUnit="FullRow" SelectedItem="{Binding SelectedRelComplete}" 
                  VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Disabled"
                  SelectionChanged="BasketDgSelectionChanged"                  
                  Drop="DataGridDrop" 

                  DragEnter="DataGridDragEnter" 
                  AllowDrop="True"
                 >

                
                
                    
                        
                            
                        
                                  

                    
                        
                            
                        

                    

                                            SelectedValueBinding="{Binding Path=RelParams.ColumnName, UpdateSourceTrigger=PropertyChanged}"
                        DisplayMemberPath="cName"
                        SelectedValuePath="cName">
                        
                            

                        
                        
                            
                        
                    

                               


            

            
                
                    
                    
                    
                    
                
                

                    
                
                
                    
                
                
                    
                
                                 

                    
                
                
                    
                
            
        
    

Edit: here is stack trace

Name:NullReferenceException Message:Object reference not set to an
instance of an object. Target:Void
RestoreAttachedItemValue(System.Windows.DependencyObject,
System.Windows.DependencyProperty) Stack: at
Microsoft.Windows.Controls.DataGridRow.RestoreAttachedItemValue(DependencyObject
objectWithProperty, DependencyProperty property) at
Microsoft.Windows.Controls.DataGridRow.SyncProperties(Boolean
forcePrepareCells) at

Microsoft.Windows.Controls.DataGridRow.PrepareRow(Object item,
DataGrid owningDataGrid) at
Microsoft.Windows.Controls.DataGrid.PrepareContainerForItemOverride(DependencyObject
element, Object item) at
System.Windows.Controls.ItemsControl.MS.Internal.Controls.IGeneratorHost.PrepareItemContainer(DependencyObject
container, Object item) at
System.Windows.Controls.ItemContainerGenerator.System.Windows.Controls.Primitives.IItemContainerGenerator.PrepareItemContainer(DependencyObject
container) at
System.Windows.Controls.VirtualizingStackPanel.InsertContainer(Int32
childIndex, UIElement container, Boolean isRecycled) at

System.Windows.Controls.VirtualizingStackPanel.AddContainerFromGenerator(Int32
childIndex, UIElement child, Boolean newlyRealized) at
System.Windows.Controls.VirtualizingStackPanel.BringIndexIntoView(Int32
index) at
Microsoft.Windows.Controls.DataGrid.ScrollRowIntoView(Object item)
at Microsoft.Windows.Controls.DataGrid.OnScrollIntoView(Object arg)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate
callback, Object args, Int32 numArgs) at
MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object
source, Delegate method, Object args, Int32 numArgs, Delegate
catchHandler) at
System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback,

Object args, Int32 numArgs, Delegate catchHandler) at
System.Windows.Threading.DispatcherOperation.InvokeImpl() at
System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object
state) at System.Threading.ExecutionContext.runTryCode(Object
userData) at
System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode
code, CleanupCode backoutCode, Object userData) at
System.Threading.ExecutionContext.RunInternal(ExecutionContext
executionContext, ContextCallback callback, Object state) at
System.Threading.ExecutionContext.Run(ExecutionContext

executionContext, ContextCallback callback, Object state, Boolean
ignoreSyncCtx) at
System.Threading.ExecutionContext.Run(ExecutionContext
executionContext, ContextCallback callback, Object state) at
System.Windows.Threading.DispatcherOperation.Invoke() at
System.Windows.Threading.Dispatcher.ProcessQueue() at
System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32
msg, IntPtr wParam, IntPtr lParam, Boolean& handled) at
MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam,
IntPtr lParam, Boolean& handled) at

MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) at
System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate
callback, Object args, Int32 numArgs) at
MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object
source, Delegate method, Object args, Int32 numArgs, Delegate
catchHandler) at
System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback,
Object args, Int32 numArgs, Delegate catchHandler) at
System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority
priority, TimeSpan timeout, Delegate method, Object args, Int32

numArgs) at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd,
Int32 msg, IntPtr wParam, IntPtr lParam) at
MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg) at
System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame
frame) at
System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Window.ShowHelper(Object booleanBox) at
System.Windows.Window.Show() at System.Windows.Window.ShowDialog()