App.WorkWith.Forms().Form("MyForm")

When I try to get the forms Sitefinty 4.x,5.0 I get an error
This method must not be called before the form description object has been loaded either through the constructor or by calling the CreateNew method.

So, what the error meant to say is that there are no forms that represent the one that you are looking for. I had the form name wrong. This could be a bug. I manually changed the name of the form for developers after I created the form. It was still the same as the original( was sf_demo - changed to Demo but it stayed as sf_demo).