You need to add a x:ClassModifier="internal" in the UserControl tag of the XAML file if you change the visibility of the class in the .cs file to internal.
The file generated from the XAML file (e.g. MyControl.g.cs) needs to match the code-behind file (e.g. MyControl.cs). x:ClassModifier allows you to control the visibility of the part declaration in the MyControl.g.cs file.