WPF实现多个RichTextBox联动 提示未处理 System.NullReferenceException（求大神们帮忙啊!!）

[sunny007sun]

背景WPF实现多个RichTextBox联动
输入数字和英文时是没有问题的，但输入中文是就提示如图错误

我的代码如下:

  MyRichTextBox Myitem = (((MyRichText.Parent as Grid).Parent as StackPanel).Parent as Grid).Parent as MyRichTextBox;



                        if (Myitem ==null)
                        {
                            return;
                        }



                        foreach (var item in sp.Children)
                        {
                            if ((item as MyRichTextBox) != Myitem)
                            {
                                (item as MyRichTextBox).richTextBox1.TextChanged -= new TextChangedEventHandler(richTextBox1_TextChanged);
                                (item as MyRichTextBox).richTextBox3.TextChanged -= new TextChangedEventHandler(richTextBox1_TextChanged);
                            }


                        }
                        for (int i = 0; i < sp.Children.Count; i++)
                        {



                            string Myxml = System.Windows.Markup.XamlWriter.Save(MyRichText.Document);

                            System.IO.StringReader MystrReader = new System.IO.StringReader(Myxml);

                            XmlReader MyXmlReader = System.Xml.XmlReader.Create(MystrReader);
                            object MyObj = System.Windows.Markup.XamlReader.Load(MyXmlReader);

                            //    FlowDocument MyFlowDocument = MyObj as FlowDocument;
                            if (MyObj as FlowDocument == null)
                            {
                                return;
                            }

                            FlowDocument MyFlowDocument = MyObj as FlowDocument;

                            if ((sp.Children[i] as MyRichTextBox) != Myitem)
                             {

                            if (MyRichText.Name == "richTextBox1")
                            {
                                (sp.Children[i] as MyRichTextBox).richTextBox1.Document = MyFlowDocument;
                            }
                            if (MyRichText.Name == "richTextBox3")
                            {

                                (sp.Children[i] as MyRichTextBox).richTextBox3.Document = MyFlowDocument;
                            }


                         }
                            MystrReader.Dispose();
                        }

                        foreach (var item in sp.Children)
                        {
                            if ((item as MyRichTextBox) != Myitem)
                            {
                                (item as MyRichTextBox).richTextBox1.TextChanged += new TextChangedEventHandler(richTextBox1_TextChanged);
                                (item as MyRichTextBox).richTextBox3.TextChanged += new TextChangedEventHandler(richTextBox1_TextChanged);
                            }
                        }

以上的代码第一个输入为中文时就会报如图错误，当第一个输入的是数字或英文后 又没有问题。
加过断点 代码中没有找到为NULL 的对象。

错误实在方法结束后报出的，所以加了try也无法捕获


唉~~~ 百思不得其解~~

[#blackheart]

原谅我没看下去的勇气，拉出去打屁屁先。。。。。。

[sunny007sun]

额~~~给点建议 也好啊~~~