ArgumentOutOfRangeException is thrown on double-clicking on empty Tree Root Node.

Steps to repro:
1. Create a new Project.
2. GameObject > 3D > Tree.
3. In Inspector > Delete Branch Group.
4. Double-click on empty Tree Root Node.

Actual Result:
ArgumentOutOfRangeException error in the cosnole.

Expected Result:
No error in the console.

Reproducible in:
2020.1.0a14, 2019.3.0f5 (fe220cd6911a), 2019.3.0a3.

Working Fine in:
2019.2.1f1.

NOTE:
-Couldn't test on 2020.1.0a17 due to https://fogbugz.unity3d.com/f/cases/1206620/
-Couldn't test on 2019.3.0a1, 2019.3.0a2 due to editor freezes on creating Tree object.

Environment:
Occurs on Windows and Mac.

Stack trace:
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.ThrowHelper.ThrowArgumentOutOfRangeException (System.ExceptionArgument argument, System.ExceptionResource resource) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.ThrowHelper.ThrowArgumentOutOfRangeException () (at <437ba245d8404784b9fbab9b439ac908>:0)
TreeEditor.TreeGroup.GetRootSpread () (at <065cf923b7714278a3c7baf218a9b966>:0)
TreeEditor.TreeEditor.FrameSelected (UnityEngine.Tree tree) (at <065cf923b7714278a3c7baf218a9b966>:0)
TreeEditor.TreeEditor.DrawHierachyNodes (TreeEditor.TreeData treeData, System.Collections.Generic.List`1[T] nodes, TreeEditor.TreeGroup group, UnityEngine.Vector2 offset, System.Single alpha, System.Single fade) (at <065cf923b7714278a3c7baf218a9b966>:0)
TreeEditor.TreeEditor.DrawHierachy (TreeEditor.TreeData treeData, UnityEngine.Renderer renderer, UnityEngine.Rect sizeRect) (at <065cf923b7714278a3c7baf218a9b966>:0)
TreeEditor.TreeEditor.InspectorHierachy (TreeEditor.TreeData treeData, UnityEngine.Renderer renderer) (at <065cf923b7714278a3c7baf218a9b966>:0)
TreeEditor.TreeEditor.OnInspectorGUI () (at <065cf923b7714278a3c7baf218a9b966>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass55_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <065cf923b7714278a3c7baf218a9b966>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)