TouchPad Error

Bellow show the error. The ERROR only show up and pause the game and I will have to unpause it to continue playing. This error only show up with I am doing some editing in the Hierarchy Tree. I am clueless to what this error means. Do you mind guiding me through it? Thanks.

NullReferenceException: Object reference not set to an instance of an object UnityEditor.TreeView.GetNewSelection (UnityEditor.TreeViewItem clickedItem, Boolean keepMultiSelection, Boolean useShiftAsActionKey) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:957) UnityEditor.TreeView.HandleUnusedMouseEventsForNode (Rect rect, UnityEditor.TreeViewItem item, Boolean firstItem) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:301) UnityEditor.TreeView.DoItemGUI (UnityEditor.TreeViewItem item, Int32 row, Single rowWidth, Boolean hasFocus) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:444) UnityEditor.TreeView.IterateVisibleItems (Int32 firstRow, Int32 numVisibleRows, Single rowWidth, Boolean hasFocus) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:568) UnityEditor.TreeView.OnGUI (Rect rect, Int32 keyboardControlID) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:504) UnityEditor.SceneHierarchyWindow.DoTreeView (Single searchPathHeight) (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:505) UnityEditor.SceneHierarchyWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:320) System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)