MS Visual Studio "Error CS0103 The name 'GridView1' does not exist in the current context"

I have an old Web App (ASP.NET Web forms) developed in VS 2015 and it runs fine under that version (see details here). Microsoft Visual Studio Community 2015 Version 14.0.25420.01 Update 3

When I open it with VS 2019 (version details below) I get the above error but it runs fine when I start it up. Microsoft Visual Studio Community 2019 Version 16.11.5

Have been searching around but no luck - very perplexing - any ideas please ?