Un-docking (RMB-click on previewer bar) generates an error in the console...

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PreviewWindow.CreateTracker () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/PreviewWindow.cs:31)
UnityEditor.InspectorWindow.OnEnable () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:108)
UnityEditor.PreviewWindow.OnEnable () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/PreviewWindow.cs:18)
UnityEditor.DockArea:OnGUI()

repro1:
-add attached or any fbx with a model and/or animation to your project
-select fbx and in the Inspector, RMB-click the preview bar to un-dock preview window... error.

repro2:
-RMB-click in the Hierarchy and create a Particle System
-in the Inspector, click the pull-down arrow of the Start Lifetime and select Curve
(curve should be visible at the bottom of the Inspector)
-RMB-click the preview bar to un-dock preview window... error.

this is a regression from 5.5.0b7 (bd91e53964bc)
bug appeared in 5.5.0b8 (dfbb819d6cdb)