Google Doc add-on to insert a template to the top of the document

Code.gs

/*
Google Doc add-on to prepend a template to the top of the document.
Based on this appending function: https://stackoverflow.com/a/38556448
Support renumbering of ordered (numbered) list.

Usage: 
Replace <GOOGLE DOC TEMPLATE URL> with the URL of the template document.
Open the target document, go to Tools > Script editor.
Paste this script into Code.gs.
Reload the target document, the add-on will be found under Add-ons menu.
The first time you use it, a pop-up will appear and request authorisation
for the access of external documents.
*/

function onOpen(e) {
  DocumentApp.getUi()
      .createAddonMenu()
      .addItem('Insert template', 'insertTemplate')
      .addToUi();
}

function insertTemplate() {
  var template_url = '<GOOGLE DOC TEMPLATE URL>';
  var mainBody = DocumentApp.getActiveDocument().getBody();
  var tempBody = DocumentApp.openByUrl(template_url).getBody();
  var listID = null;
  var lastListID = null;
  var cursor = DocumentApp.getActiveDocument().getCursor().getElement(); 
  var cursorID = cursor.getParent().getChildIndex(cursor);
  for(var i = 0;i<tempBody.getNumChildren();i++){
    var element = tempBody.getChild(i).copy();  
    var type = element.getType();                
    if(type == DocumentApp.ElementType.TABLE){
      mainBody.insertTable(cursorID+i, element);}
    else if(type == DocumentApp.ElementType.PARAGRAPH){
      mainBody.insertParagraph(cursorID+i, element);}
    else if(type == DocumentApp.ElementType.LIST_ITEM){
      if(listID==null)  // First list item
        listID = mainBody.insertListItem(cursorID+i, 'temp');
        lastListID = listID;
      var glyph = element.getGlyphType();
      var item = mainBody.insertListItem(cursorID+i, element).setListId(listID).setGlyphType(glyph);
    }
    else if(type == DocumentApp.ElementType.PAGE_BREAK)
      mainBody.insertPageBreak(cursorID+i, element);
  }
  mainBody.removeChild(listID);
  
}