To reproduce:
1. Open a new project.
2. Go to: Edit -> Preferences -> Colors.
3. Change any color, or press "Use Defaults".

Actual result: Errors appear.
Expected result: No errors

Reproducible with: 5.6.0b5, 5.6.0f1, 2017.1.0a3
Not reproducible: 5.5.2p2, 5.6.0b4

Error message:
InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:321)
UnityEngine.GUILayout.EndVertical () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:317)
UnityEditor.PreferencesWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesWindow.cs:304)
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 C:/buildslave/unity/build/Editor/Mono/HostView.cs:262)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:255)
UnityEditor.HostView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:85)
UnityEditor.HostView:OnGUI()