title: WPF application crash
body: <p>I am working on a WPF application with a TabControl (3 tabs with textBlocks, labels and dataGrids).  The application auto-refreshes the data on these tabs using background threads.  Everything works great except for the fact that if I switch tabs when one of the background processes fires, it throws an "Object reference not set to the instance of an object" error and crashes the application.  I have set up DispatcherUnhandledException to handle any unhandled exception on the UI thread.  The stack trace does not provide much data other than stating the fact that the error is coming from the TabControl and possibly the objects associated with the selected tab.  My question is this:  Is their inherent issues with the WPF TabControl that may cause this behavior?  I have, as previously mentioned, set up as much error handling as I know how but all that is received is the "Object reference..." error.</p>
  
  <p>Stack trace:</p>
  
  <p>at System.Windows.Controls.ItemContainerGenerator.MoveToPosition(GeneratorPosition position, GeneratorDirection direction, Boolean allowStartAtRealizedItem, GeneratorState&amp; state)
  at System.Windows.Controls.ItemContainerGenerator.Generator..ctor(ItemContainerGenerator factory, GeneratorPosition position, GeneratorDirection direction, Boolean allowStartAtRealizedItem)
  at System.Windows.Controls.ItemContainerGenerator.System.Windows.Controls.Primitives.IItemContainerGenerator.StartAt(GeneratorPosition position, GeneratorDirection direction, Boolean allowStartAtRealizedItem)
  at System.Windows.Controls.VirtualizingStackPanel.MeasureOverride(Size constraint)
  at Microsoft.Windows.Controls.Primitives.DataGridRowsPresenter.MeasureOverride(Size constraint) in C:\dd\WPF_1\src\wpf\src\ControlsPack\WPFToolkit\DataGrid\Microsoft\Windows\Controls\Primitives\DataGridRowsPresenter.cs:line 118
  at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
  at System.Windows.UIElement.Measure(Size availableSize)
  at System.Windows.ContextLayoutManager.UpdateLayout()
  at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
  at System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork()
  at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
  at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
  at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
  at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
  at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, 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)
  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&amp; handled)
  at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean&amp; handled)
  at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
  at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
  at System.Windows.Threading.Dispatcher.WrappedInvoke(Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
  at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
  at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg)
  at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
  at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG&amp; msg)
  at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
  at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
  at System.Windows.Threading.Dispatcher.Run()
  at System.Windows.Application.RunDispatcher(Object ignore)
  at System.Windows.Application.RunInternal(Window window)
  at System.Windows.Application.Run(Window window)
  at System.Windows.Application.Run()
  at salesforceConnect.App.Main() in C:\Users\jfeltis\Documents\Expression\Blend 3\Projects\salesforceConnect\salesforceConnect\obj\Debug\App.g.cs:line 0</p>
  
  <p>All of this is gibberish to me.  Can someone smarter than myself take a look and, hopefully, give me some direction?  Thank you.</p>