Steps to reproduce:
1. Open the attached project "repro815812.zip"
2. Select "PARENT" prefab in the Project window.
3. Note the warning in the console

Expected results: No warning appears in the console
Actual results: Warning appears in the console "SendMessage cannot be called during Awake, CheckConsistency, or OnValidate UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)"

Reproducible with: 2019.3.14f1(7.3.1), 2020.1.0b10(8.1.0), 2020.2.0a12(8.1.0)
Can't test with: 2018.4 due to errors

Note: Check the edit field to see "Relevant information from the customer"

To not reproduce the issue do the following:
1. Relaunch the project again
2. Select "CHILD" prefab in the Project window
3. Select "PARENT" prefab in the Project window
4. Note that the warning does not appear if "CHILD" prefab was selected first