To add a custom button, I use the following code in the Page_Load of my web form:

WebControl toolctrl4 = Editor1.CreateCommandButton("send","send.gif","Insert My Custom Text");
toolctrl4.Attributes["onclick"]="document.Form1.submit();";
int pos = Editor1.ToolControls.IndexOf("Save")+1;
Editor1.InsertToolControl(pos,"send",toolctrl4);

The button works fine on the first view of the page, but when there is a PostBack the page fails:
Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.

...because of this custom button.

how do I resolve this problem?