How to reproduce:

1. Open attached project
2. Select the Editor/SelectMe.asset file in the Project window
- Note the error thrown in the console:
GetAssetPath is not allowed to be called during serialization, call it from Awake or Start instead. Called from script 'TestContainer' on game object 'SelectMe'.
See "Script Serialization" page in the Unity Manual for further details.
UnityEditor.AssetDatabase:GetAssetPath(Object)
TestContainer:OnBeforeSerialize() (at Assets/Editor/TestContainer.cs:9)
UnityEditor.Editor:OnInspectorGUI()
TestWindow:OnInspectorGUI() (at Assets/Editor/TestWindow.cs:7)
UnityEditor.DockArea:OnGUI()
- Reproduced in Version 5.4.0b7 (83df6fa5e23f)
- Not reproducible in Version 5.3.3p1 (828971bd30e1)
- Message doesn't make sense for ScriptableObject since it has no Awake or Start messages.

- By Design: The Unity API is no longer allowed to be called from serialisation under any circumstances and in the future there may be changes to serialisation that make this use case run on a separate thread.