1. install 4.6b6 on windows from: http://teamcity.unity3d.com/viewLog.html?buildId=697722&tab=artifacts&buildTypeId=bt3886
2. when installed create new project

Result: a pop up error appears saying that it can't load the layout giving options of quit and continue.

3. select continue

result: project opens but you get two errors:

Error while reading window layout: no main window found
UnityEditor.WindowLayout:LoadWindowLayout(String, Boolean)

Failed to load window layout: System.Exception: Exception of type 'System.Exception' was thrown.

Note: unable to repro unless you re install is likely. The errors can be cleared.