How do I open new item form from script editor

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SharePoint2013

I am trying to open the new item form in a modal dialog. The following code works perfectly when used in SharePoint-hosted app, but does for some reason not work in a script editor web part on a SharePoint page. My guess is that the script file doesn't load properly.

function openNewItemDialog() {

       var options = {
           url: appWebUrl + "/Lists/ListName/NewForm.aspx",
           width: 500,
           height: 500,
           title: "Form title"
       };
       SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.showModalDialog', options);
}

I would appreciate som help.

Thanks