Styling_Buttons_FlatIconButton_3_162224436' TargetType does not match type of element 'Styling_Buttons_FlatIconButton_4_162224436'

Question

I\'ve a solution with 2 projects in it.

1. Styling (WPF Control Library, .NET 4.7.1, x64)
2. Styling.Test (WPF Application, .NET 4.7.1, x64)

The