Getting the following error in the Console when clicking in an empty Animator window.

repro:
-start Unity (repro only happens on a fresh start)
-create a new Project
(needs to be a new empty project, no Animator controller)
-open the Animator window
-click the Animator window canvas (no controller selected)... error...

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.AnimationBlendTree.GraphGUI.DoBackgroundClickAction () (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationBlendTree/GraphGUI.cs:348)
UnityEditor.Graphs.AnimatorControllerTool.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1021)
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)

repro in...
Version 5.6.0f1 (95292252f04a)
Branch: 5.6/staging

not repro in Unity5.5.2p2