Steps to reproduce:
1. Create a project
2. Drag a window to second display (repeat until there is error)
4. Observe the error message:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorWindow.set_position (Rect value) (at C:/buildslave/unity/build/Editor/Mono/EditorWindow.cs:843)
UnityEditor.DockArea.DragTab (Rect pos, UnityEngine.GUIStyle tabStyle) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:645)
UnityEditor.DockArea.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:339)

Expected result: Positive/Negative button's tooltips should not be inverted
Actual result: Positive/Negative button's tooltips are inverted

Reproduced with: 5.6.0b6, 5.6.0b7
Not reproduced with: 5.3.7p4, 5.4.4p2, 5.5.1p2, 5.6.0b5
Regression introduced in 5.6.0b6