BadLocationException When SetContentType (text / Html) In JTextPane

I want to setContentType("text/html") to be able to use html in JTextPane but I get BadLocationException when I try to remove the first line.

Here is the code

    setContentType("text/html") ;
    try {
         Element root = getDocument().getDefaultRootElement();
         Element firstLine = root.getElement(0);
        getDocument().remove(firstLine.getStartOffset(), firstLine.getEndOffset());
        }
    catch (BadLocationException e) { }