Visual studio 2019 windows forms designer

As of version 16.8.0 Preview 2.1 I have found that I have to turn off this setting and restart and add my form. Once added, I can turn it back on, restart and work.

To enable the designer, download and install the Windows Forms .NET Core Designer VSIX package.

See article: https://devblogs.microsoft.com/dotnet/introducing-net-core-windows-forms-designer-preview-1/

I had the same problem. The following steps helped me

1. Remove any other classes than the Form class in the yourformname.cs file
2. Unload the project and right click on the project and choose edit project
3. Make sure yourformname form has the SubType set to form <SubType>Form</SubType> and save the file
4. Reload the project