Guard ReferenceType consumption for ExternalResource

Author: captainsafia

src/Microsoft.OpenApi/Models/OpenApiReference.cs

@@ -200,8 +200,11 @@ private string GetExternalReferenceV3()
                 {
                     return ExternalResource + "#" + Id;
                 }
-
-                return ExternalResource + "#/components/" + Type.Value.GetDisplayName() + "/"+ Id;
+                
+                if (Type.HasValue)
+                {
+                    return ExternalResource + "#/components/" + Type.Value.GetDisplayName() + "/"+ Id; 
+                }
             }
 
             return ExternalResource;