Why TypeError: document.getElementById() is null
===

Why the following code generates 'TypeError: document.getElementById("docPrint") is null'

    var printwindow = window.open('', '', 'fullScreen=no');
    printwindow.document.write('<iframe id="docPrint" width="100%" height="100%" src="http://localhost:8080/hiring/docs/Keneth _1340800082258/Keneth _resume_1340800082258.pdf"></iframe>');
    printwindow.self.focus();								document.getElementById('docPrint').focus();
    document.getElementById('docPrint').contentWindow.print();

