More precise repro instructions:
1.) Added a Bounding Box Follower Compnent.
2.) Put the Player (Skeleton Animator) in the Skeleton Renderer.
3.) In the "Slot" option I select the hit boxes I'm looking for, and the Attachment Names show up. (The bounding boxes I created in Spine.) There is an option to add bone follower that also appears.
4.) This is when I get this error: ArgumentException: Getting control 2's position in a a group with only 2 controls when doing Repaint Aborting.
ArgumentException: Getting control 2's position in a group with only 2 controls when doing Repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:656)
UnityEngine.GUILayoutUtility.DoGetRect (Single minWidth, Single maxWidth, Single minHeight, Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:401)
UnityEngine.GUILayoutUtility.GetRect (Single minWidth, Single maxWidth, Single minHeight, Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:390)
UnityEditor.EditorGUILayout.GetToggleRect (Boolean hasLabel, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7158)
UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/ScriptAttributeGUI/PropertyHandler.cs:191)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7119)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7101)
Spine.Unity.Editor.BoneFollowerInspector.OnInspectorGUI () (at Assets/3rdParty/Spine/spine-unity/spine-unity/Editor/BoneFollowerInspector.cs:83)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1231)
UnityEditor.DockArea😮nGUI()