Object reference not set to an instance of an object.

Server Version : 1.4.1.27 Studio Version : 1.4.1.27 Email Address : BB Steps to follow : Opened a nested workflow. at Dev2.Studio.Core.DebugStateTreeViewItemViewModel.BuildBindableListFromDebugItems(IEnumerable`1 debugItems, ICollection`1 destinationList) at Dev2.Studio.Core.DebugStateTreeViewItemViewModel.Initialize(IDebugState content) at Dev2.Studio.Core.DebugTreeViewItemViewModel`1.set_Content(TContent value) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.CreateChildTreeViewItem(IDebugState content) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.AddTreeViewItemToRootItems(IDebugState content) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.AddItemToTreeImpl(IDebugState content) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.<>c__DisplayClass158_2.b__3() at System.Windows.Threading.Dispatcher.Invoke(Action callback, DispatcherPriority priority, CancellationToken cancellationToken, TimeSpan timeout) at System.Windows.Threading.Dispatcher.Invoke(Action callback) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.AddItemToTree(IDebugState content) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.Append(IDebugState content) at Dev2.Studio.ViewModels.Diagnostics.DebugOutputViewModel.<.ctor>b__38_0(DebugWriterWriteMessage msg) at System.Reactive.AnonymousSafeObserver`1.OnNext(T value) at System.Reactive.Concurrency.ObserveOn`1.ς.OnNextPosted(Object value) 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)