There is no other workaround ?

If your solution contains a setup project and this setup project has been
built at least once, you get an Assembly Load error when you run the
Inheritance Picker in order to inherit from an existing form in your main
project.

This problem occurs because the setup project generates several executable
files - such as InstMsiA.exe, InstMsiW.exe, and Setup.exe - that aren't
valid .NET assemblies. However, the Inheritance Picker interprets these
files as assemblies and attempts to load them, which results in a failure.

The only solution to this problem is removing the setup project from the
solution, and re-add it to the solution only when you're about to package
the solution.