Right click on the Asset Preview window in the Inspector to have it undocked. The following error message is printed in the Console:
"NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PreviewWindow.CreateTracker () (at C:/buildslave/unity/build/Editor/Mono/Inspector/PreviewWindow.cs:31)
UnityEditor.InspectorWindow.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:108)
UnityEditor.PreviewWindow.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/Inspector/PreviewWindow.cs:18)
UnityEditor.DockArea:OnGUI()"

This does not happen in Unity 5.4, so it's a regression introduced in Unity 5.5