Optimized code

Src/Asp.NetCore2/SqlSugar/Abstract/QueryableProvider/QueryableHelper.cs

@@ -687,12 +687,15 @@ namespace SqlSugar
                     foreach (var it in managers)
                     {
                         var manager = it as NavigatManager<TResult>;
+                        if (manager != null)
+                        {
                             manager.RootList = result;
                             manager.Execute();
                         }
                     }
                 }
             }
+        }
 
         private void SelectNavQuery<TResult>(List<TResult> result, List<object> managers)
         {