title: Object has been destroyed when open secondary/child window in electron js
body: <p>In a main window, when a button is clicked, the second/child window will popped up via an ipc call. 
  This works when open the pop window on the first time. If I closed the pop window and reopen it again, I will get this error:</p>
  
  <blockquote>
    <p>Uncaught Exception: Error: Object has been destroyed at Error (native)
    at EventEmitter. (/home/xxxx/electron/fin/main.js:36:21) at emitOne
    (events.js:96:13) at EventEmitter.emit (events.js:188:7) at
    EventEmitter.
    (/home/xxxx/electron/fin/node_modules/electron-prebuilt/dist/resources/electron.asar/browser/api/web-contents.js:156:13)
    at emitTwo (events.js:106:13) at EventEmitter.emit (events.js:191:7)</p>
  </blockquote>
  
  <pre><code>#main.js on app ready:
  mainWindow = new BrowserWindow({width: 800, height: 600})
  
  
    mainWindow.loadURL(`file://${__dirname}/index.html`)
  
  
    mainWindow.webContents.openDevTools()
  
  
    mainWindow.on('closed', function () {
  
      mainWindow = null
    })
  
    let popWindow = new BrowserWindow({parent: mainWindow, width: 450, height: 450, show: false});
    popWindow.loadURL(`file://${__dirname}/app/pop.html`);
    popWindow.webContents.openDevTools();
  
    ipc.on('toggle-popwindow', function(){
           popWindow.show();    
    });
  </code></pre>
  
  <p>And when I add hide() in the 'closed' method:</p>
  
  <pre><code>popWindow.on('closed', function (event) {
  popWindow.hide();
  });
  </code></pre>
  
  <p>I get this:</p>
  
  <pre><code>Uncaught Exception:
  Error: Object has been destroyed
  </code></pre>
  
  <p>What is the problem?</p>