Steps to reproduce:

1. Open Unity 5.5 (trunk) on Windows;
2. Double click on 'Sprite' file in "Assets" folder of 'Project' tab.

Actual result:

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Peek () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:321)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:289)
UnityEditor.DockArea.EndOffsetArea () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:449)
UnityEditor.DockArea.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:358)

is shown in the console.
See attached gif video and screenshot.

Expected result:
This error should not be shown.

Regression introduced in Unity 5.5.

Notes:
- Reproducible only on Windows 10 (checked);
- Not reproducible in OSX 10.11.4;
- Reproducible in 5.5.0a1 (58d06184d89d) trunk;
- Not reproducible in 5.4.0b21, 5.3.5p1;
- Reproducible for any imag file that may be opened by external application.