Microsoft Visual Studio C# Error Cannot implicitly convert type 'System.Wi​ndows.Form​s.RichText​Box' to 'string'

I want to make a HTML editor in C# in Visual Studio with two forms.One form contains the web browser and the other the text box.I got the following error in code: Cannot implicitly convert type 'System.Windows.Forms.RichTextBox' to 'string' . Form1 is the form where is located the richtextbox.

        private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            Form1 m = new Form1();
            webBrowser1.DocumentText = m.codeBox1;
        }