NullReferenceException: Object reference not set to an instance of an object
UnityEditor.AudioSourceInspector.OnDisable () (at C:/buildslave/unity/build/Editor/Mono/Inspector/AudioSourceInspector.cs:165)
UnityEditor.AssetDatabase:DeleteAsset(String)
ES2EditorStartup:Cleanup()
ES2EditorStartup:.cctor()
UnityEditor.EditorAssemblies:SetLoadedEditorAssemblies(Assembly[])
Looks like there's a bug at Unitys end affecting cleanup scripts. However, we're removing the need for cleanup scripts in the next version anyway, so this will no longer be a problem.
In the meantime the error should be harmless and doesn't affect anything at runtime.