Disable Web Browser Show Document Modified Prompt

I have created an HTML editor. In my application, I have a toolbar to toggle visiblity of a webbrowser control and a multiline textbox control to show HTML page and HTML source respectively.

All is working fine except let's say I input "Hello World" as input in HTML mode, then click the toggle button to switch to Text Mode, I will see the HTML source correctly. However, when I tried to toggle back to HTML mode, I will get a prompt telling me "document has been modified". How do I disable this prompt from appearing?

I shall attach the screen for the "dialog" in a short while.

Below is my code fragment.
        private void viewButton_Click(object sender, EventArgs e)
        {
            if (viewButton.Text == "HTML")
            {
                txtEditor.Text = webBrowser1.DocumentText;
                txtEditor.Visible = true;
                webBrowser1.Visible = false;
                viewButton.Text = "TEXT";
            }
            else
            {
                webBrowser1.DocumentText = txtEditor.Text;
                txtEditor.Visible = false;
                webBrowser1.Visible = true;
                viewButton.Text = "HTML";
            }
        }