How to check if javascript element id exist?

[nicholaslee]

if (ui->webView->page()->mainFrame()->evaluateJavaScript(QString("document.getElementById('ContentPlaceHolder1_ErrorTextLabel').value"))!=null)
        {
}

I want to make a function that does commands whenever the user makes an invalid log in. So I made a getElementById such that whenever the error text label exist, i can carry out other commands but qt is giving me the error ['null' was not declared in this scope]
Is it a syntax or declaration error?
Anyone can help me with this? Thanks!