Could not reproduce this again. But what I did was to delete an Audio Group and then rename another - result was the following error messages in Console:

Style.Draw may not be called with GUIContent that is null.
UnityEngine.Debug:Internal_Log(Int32, String, Object)
UnityEngine.Debug:LogError(Object)
UnityEngine.GUIStyle:Draw(Rect, GUIContent, Int32, Boolean) (at C:/Users/NEVIN/Desktop/audiomixer/unity/artifacts/EditorGenerated/GUIStyleBindings.cs:717)
UnityEditor.EditorGUI:ButtonMouseDown(Int32, Rect, GUIContent, GUIStyle) (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/GUI/InternalEditorGUI.cs:35)
UnityEditor.AudioSourceInspector:AudioMixerPickerGUI() (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/Inspector/AudioSourceInspector.cs:331)
UnityEditor.AudioSourceInspector:OnInspectorGUI() (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/Inspector/AudioSourceInspector.cs:418)
UnityEditor.InspectorWindow:DrawEditor(Editor, Int32, Boolean, Boolean&, Rect&, Boolean) (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/Inspector/InspectorWindow.cs:1115)
UnityEditor.InspectorWindow:DrawEditors(Editor[]) (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/Inspector/InspectorWindow.cs:934)
UnityEditor.InspectorWindow:OnGUI() (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/Inspector/InspectorWindow.cs:295)
System.Reflection.MonoMethod:InternalInvoke(Object, Object[], Exception&)
System.Reflection.MonoMethod:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo) (at /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
System.Reflection.MethodBase:Invoke(Object, Object[]) (at /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView:Invoke(String, Object) (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/GUI/DockArea.cs:241)
UnityEditor.HostView:Invoke(String) (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/GUI/DockArea.cs:234)
UnityEditor.DockArea:OnGUI() (at C:/Users/NEVIN/Desktop/audiomixer/unity/Editor/Mono/GUI/DockArea.cs:671)