Why the OuterHTML feature leads to an Error

I have been trying to change the HTML code of a select tag in an HTML code but whenever I do that the

"Object reference not set to an instance of an object"

error occurs.This is the C# code:

public void SetDefaultValue(string ControlID,string Value)
{
    HtmlDocument doc = webBrowser1.Document;
    HtmlElement list = doc.GetElementById(ControlID);
    list.OuterHtml = Value;         
}
    private void Form1_Load(object sender, EventArgs e)
    {            
        htmlLoadWebbrowser1.SetDefaultValue("D8876943-5861-4D62-9249-C5FEF88219FA", " item 3");            
    }

The function is supposed to change the inner code of an HTML page,it receives the ID of the control by "controlID" and generates the inner code by "Value".

I suspect that I need to call it elsewhere,the problem is I do not know where

Thank you.