What Happened:
On rotating node of a nested branch group of a tree, the editor throws null reference exception

Steps To Reproduce:
1. 3D Object > Create Tree
2. Tree > Select branch group in tree creation window > Click on "Add Branch" icon
3. Click on the rotation node on the branch, in the scene view.
5. Observe that clicking on brach node rotate icon throws " NullReferenceException" error.

Actual Result:
Selecting the rotation icon shouldn't throw any error.

Expected Result:
Selecting the rotation icon on the branch throws null reference error.

Frequency:
Always

Steps To Reproduce:

1. Create a 3d game object of type tree.
2. Select the branch icon on the inspector.
3. Click on the 'Add Brach' icon.
4. Click on the rotation icon on the branch in the scene view.
5. Observe that clicking on brach rotating icon of a branch on a tree throws " NullReferenceException" error.

Actual Result:
Selecting the rotation icon on the branch throws null reference error.

Expected Result:
Selecting the rotation icon shouldn't throw any error.

Frequency:
Always

Environment:
2017.3.0b11
Occurring on windows & Mac

Working on:
Unity 5.6.3f1, 2017.1.2f1, 2017.1.2p3

Reproducible with:
2017.2.0a1, 2017.2.0f1, 2017.2.0f3, 2017.3.0b11

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
TreeEditor.TreeEditor.OnSceneGUI () (at /Users/builduser/buildslave/unity/build/Extensions/TreeEditor/TreeEditor/TreeEditor.cs:1076)
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.SceneView.CallOnSceneGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2507)
UnityEditor.SceneView.HandleSelectionAndOnSceneGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1833)
UnityEditor.SceneView.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1663)
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:295)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)