My code

const electron = require('electron');
const url = require('url');
const path = require('path');

const {app , BrowserWindow , Menu} = electron;

let mainWindow;
let addWindow;

//Listen for the app to be ready
app.on('ready' , function(){
    //Create new window
    mainWindow = new BrowserWindow({});
    //Load HTML file into the main window
    mainWindow.loadURL(url.format({
        pathname: path.join(__dirname , 'mainWindow.html'),
        protocol: 'file:',
        slashes:true
    }));

    //Build menu from template
    const mainMenu = Menu.buildFromTemplate(mainMenuTemplate);

    //Insert the menu
    Menu.setApplicationMenu(mainMenu);
});

//Handle CreateAddWindow function
function CreateAddWindow()
{
   //Create new window
    addWindow = new BrowserWindow
    ({
        width: 200,
        height: 300,
        title: 'Add to shopping list'
    });
    //Load HTML file into the main window
    addWindow.loadURL(url.format({
    pathname: path.join(__dirname , 'addWindow.html'),
    protocol: 'file:',
    slashes:true
    }));
}


const mainMenuTemplate =
[
    {
        label:'File',
        submenu:
        [
            {
                label:'Add Item',
                CreateAddWindow()
            },
            {
                label: 'Clear Items'
            },
            {
                label:'Quit',
                accelerator: process.platform == 'darwin' ? 'Command+Q' : 'Ctrl+Q',
                click()
                {
                    app.quit();
                }
            }
        ]
    }
]