Hierarchy not showing anything, getting KeyNotFoundException

My Unity project seems seriously buggered.

My scene is loaded but my hierarchy is empty. It does this in EVERY scene, not just one. It looks like this:

I'm getting tons of errors, but the one one that is repeating is this:

 KeyNotFoundException: The given key was not present in the dictionary.
 System.Collections.Generic.Dictionary`2[System.String,UnityEditor.BaseHierarchySort].get_Item (System.String key) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150)
 UnityEditor.SceneHierarchyWindow.SortMethodsDropDown () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:645)
 UnityEditor.SceneHierarchyWindow.DoToolbar () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:560)
 UnityEditor.SceneHierarchyWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:318)
 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)

And I'm occasionally getting this one too:

 ArgumentException: Getting control 0's position in a group with only 0 controls when doing Repaint
 Aborting
 UnityEngine.GUILayoutGroup.GetNext ()
 UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType)
 UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options)
 UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options)
 UnityEditor.SceneHierarchyWindow.DoToolbar () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:513)
 UnityEditor.SceneHierarchyWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:318)
 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)

I suspect it's an Editor class or something that's gone out of whack - but can anyone help me understand the error and get to the bottom of it?

Cheers!