To reproduce:
- Import attached "TestWindow.unitypackage" and launch "test" scene
- Go to "Test" in top menu bar an select "CurveField Test"
- Dock the "Test Window" somewhere in the editor
- Click on the curve in the Test Window and observe that the Curve Editor appears. Also observe that there are no errors in Console
- Reimport all and observe the following errors in Console:

"m_PathToGUID.count(goodPath) == 0"

- Click on the curve field in Test Window and observe the following error messages:

"Style.Draw may not be called with GUIContent that is null.
UnityEditor.HostView:OnGUI()"

Could reproduce on the latest version 5.0.0a7 (99a2a2d4bb7d)