Reference to type 'Assembly' claims it is defined in 'System.Runtime', but it could not be found

Already posted a question related to this issue, but the solution there is not solving the issue now for another project.

My error:

Severity Code Description Project File Line Suppression State Error CS7069 Reference to type 'Assembly' claims it is defined in 'System.Runtime', but it could not be found

I have this problem after updating xamarin forms to 3.4.0.1008975 from 2.5.0.121934. Problem is only on the UWP part. Android and IOS apps are working fine.

Error lines:

var rendererAssemblies = new List<Assembly>
   {
       typeof(ImageCircleRenderer).GetTypeInfo().Assembly
   };
rendererAssemblies.AddRange(Rg.Plugins.Popup.Popup.GetExtraAssemblies());
Rg.Plugins.Popup.Popup.Init();
Xamarin.Forms.Forms.Init(e, rendererAssemblies);

Another solution found on this thread. The solution on this thread is downgrading the xamarin forms to version 2.5.1.527436 in all the projects. But I can't do that in my project, because I am using some features that are available in the latest xamarin forms.

So how can I fix this issue without degrading the xamarin forms?