Reproduction steps:
1. Open the attached project "Aspect_Ratio_Fitter_Bug.zip";
2. Open the test scene;
3. Select the Canvas game object;
4. In the Aspect Ratio Fitter, change the Aspect Mode from "None" to "Fit in Parent".

Actual results: observe the following error in the console:
SendMessage cannot be called during Awake, CheckConsistency, or OnValidate
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)

Reproduced on: 2017.1.0b4, 2017.1.3p2, 2017.2.0b4, 2017.2.1p4, 2017.3.2f1, 2018.1.0b12, 2018.2.0a5.
Not reproduced on: 5.6.5p1, 2017.1.0b3, 2017.2.0b3.