How to reproduce:
1. Open the attached project "NGUI_Bug.zip"
2. Open NGUI>Open>Font Maker
3. Select "Font Source" as the Font Data
4. Select "Font Texture" as the Texture
5. Select "Atlas Asset" as the Atlas for the Output
6. Click the "Create the Font" button
7. Click "Save"
Expected results: No errors thrown in the Console window
Actual results: "Serialization not implemented for type MonoBehaviour" thrown in the Console window
Reproducible with: 2022.2.20f1, 2023.1.0b17, 2023.2.0a14
Not reproducible with: 2020.3.48f1, 2021.3.25f1, 2023.2.0a15
Reproducible on: Windows 10