Steps to reproduce:

1. Open attached project "BugReportProject_HierarchyBug.rar"
2. Open scene "TestScene"
3. Click on hierarchy and press ctrl + a to select all object
4. Press right arrow key to unfold hierarchy
5. Notice exception:
ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
System.Collections.Generic.List`1[UnityEditor.TreeViewItem].get_Item (Int32 index) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:633)
UnityEditor.GameObjectTreeViewDataSource.GetItem (Int32 row) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/GameObjectTreeViewDataSource.cs:134)
UnityEditor.TreeView.GetItemAndRowIndex (Int32 id, System.Int32& row) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:771)
UnityEditor.TreeView.KeyboardGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:859)
UnityEditor.TreeView.OnGUI (Rect rect, Int32 keyboardControlID) (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeView.cs:520)
UnityEditor.SceneHierarchyWindow.DoTreeView (Single searchPathHeight) (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:585)
UnityEditor.SceneHierarchyWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:399)
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/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)

Not reproducible: 5.2.4f1
Reproduced with: 5.3.6p1, 5.4.0f3, 5.5.0a5