Error in value of GetExtension code

Hello,

 When I set the value of GetExtension, the variable gets null.         

 foreach (SOLine soline in listOfLine1)
            {
                SOLine line = new SOLine();

                //…. code

                var lineExt = SOGraph.Transactions.Current.GetExtension<Objects.SO.SOLineExt>();

                lineExt.UsrMargeLine = GetPercentMargin(line);

                SOGraph.Transactions.Insert(line);
                SOGraph.Actions.PressSave();

            }

Best regards.