fixes#330-Yield validation err when an invalid security req key found

Author: senthilkumarmohan

src/Microsoft.OpenApi.Readers/Services/OpenApiReferenceResolver.cs

@@ -108,11 +108,14 @@ public override void Visit(OpenApiSecurityRequirement securityRequirement)
             {
                 ResolveObject(scheme, (resolvedScheme) =>
                 {
-                    // If scheme was unresolved
-                    // copy Scopes and remove old unresolved scheme
-                    var scopes = securityRequirement[scheme];
-                    securityRequirement.Remove(scheme);
-                    securityRequirement.Add(resolvedScheme, scopes);
+                    if (resolvedScheme != null)
+                    {
+                        // If scheme was unresolved
+                        // copy Scopes and remove old unresolved scheme
+                        var scopes = securityRequirement[scheme];
+                        securityRequirement.Remove(scheme);
+                        securityRequirement.Add(resolvedScheme, scopes);
+                    }
                 });
             }
         }