It is used in an items control containing other field model.So it is pretty much implicit datatamplte.The "expected parent" is simply a contentcontrol. 

Oddly enough we dont get any stacktrace of the error.

Also it is used in an items control with wrap panel as a panel template

I switched to a WrapPanel as the ItemsPanel but that didn't seem to change anything.  Can you update the sample I attached in my last update to reproduce the exception and then send it back to me so I can debug it?

Trying to reproduce the issue.

Doesnt occur by simply putting the into a template.

So havnt found a way to reproduce it in a simple project.

The only thing new i found is that the exception only occur if I click on a an area with text. The adventure got a lot stranger. 

So ill keep trying to repoduce it. 

Found the issue but do not know a way around unless I handle DispatcherUnhandled exception. This is the stack trace . 

at System.Windows.LogicalTreeHelper.GetParent(DependencyObject current)
at Infragistics.DragDrop.DragDropManager.GetUIElementParent(DependencyObject element)
at Infragistics.DragDrop.DragDropManager.OnMouseLeftButtonDown(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
at System.Windows.UIElement.OnMouseDownThunk(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.ContentElement.RaiseTrustedEvent(RoutedEventArgs args)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Interop.HwndSource.InputFilterMessage(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 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

So the element has a parent XamDataGridCell usign the DragDropManager. Not understanding why it i triggered but handling the exception seems the only workaround I can think of

Hi Etienne,

I logged a dev issue for this so our engineers can make some changes around that code to account for null values getting passed to the GetParent() method.  While I was not able to reproduce this locally the GetUIElementParent() method is very small and there are only two places where LogicalTreeHelper.GetParent() are called so I figured it would be ok to log this along with the call stack you provided.  The dev issue ID is 247206.

For now, continue to use your workaround.  I will open a private support case for you soon where I will link to this dev issue so you can track its status.  Be aware that development has stopped for 16.2 and below so any fix that is made will be in 17.1 and above.  Once the case is created I will update this thread and let you know.

We host elements in xamdatagrid styled as a list view.

Elements support drag and drop using drag drop manager. We use the drag drop manager to merge entries in the grid. 

So at some point upon clicking an element on the grid inside the editor I've seen the drag drop procedure is activated on the MouseLeftButtonDown but unsuccessfully find a parent.

Hmm.  I actually do see the possibility for an ArgumentNullException in the DragDropManager.GetUIElementParent method but I don't know how it could actually happen assuming the element that the DragSource is attached to is in the visual tree and I think it should be since the MouseLeftButtonDown event is firing which is what calls GetUIElementParent.

Speaking of the Infragistics DragDropManager, how are you using it in your app and how does it relate to the XamMultiColumnComboEditor?  Based on the call stack it seems like an important part that my sample is missing so I'd like to try adding it in the same way it is used in your app.

version used is 16.1.20161.1000.