TargetFramework update,

reference updates and added tests for Viewer

Tests/Models/Binder/AppSettings.cs

@@ -58,7 +58,9 @@ public class AppSettings
     public static Models.AppSettings Get(IConfigurationRoot configurationRoot)
     {
         Models.AppSettings result;
-        AppSettings appSettings = configurationRoot.Get<AppSettings>();
+        AppSettings? appSettings = configurationRoot.Get<AppSettings>();
+        if (appSettings is null)
+            throw new NullReferenceException(nameof(appSettings));
         result = Get(appSettings);
         return result;
     }