Steps to reproduce :

- create an asset
- add a label to it and notice the error

*Note:
Regression from 2017.b5.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.LabelGUI.DrawLabelList (Boolean partiallySelected, Single xMax) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/LabelGUI.cs:198)
UnityEditor.LabelGUI.OnLabelGUI (UnityEngine.Object[] assets) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/LabelGUI.cs:177)
UnityEditor.InspectorWindow.DrawPreviewAndLabels () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:764)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:397)
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)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
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:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:272)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:265)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:232)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)