Fix passing WebContents to the window option

[kondoumh]

This is a fix that allows you to specify WebContents for the window option of the contextMenu.
I want to receive a WebView dom-ready event from the renderer process and set WebContents in the main process to create a context menu. Like this.

ipcMain.on("webview-ready", (e, url) => {
  const content = webContents.getAllWebContents().find(c => c.getURL() === url);
  contextMenu({
    window: content,
    prepend: (defaultActions, parameters, mainWindow) => [
      {
        label: 'Rainbow',
        visible: parameters.mediaType === 'image'
      },
      {
        label: 'Search Google for “{selection}”',
        visible: parameters.selectionText.trim().length > 0,
        click: () => {
          shell.openExternal(`https://google.com/search?q=${encodeURIComponent(parameters.selectionText)}`);
        }
      }
    ]
  });
});

Currently, when I specify WebContents for window, I get the following error:

Uncaught Exception:
TypeError: Cannot read property '_events' of undefined
at _addListener (events.js:350:19)
at addListener (events.js:406:10)
at module.exports (/xxxx/node_modules/electron-context-menu/index.js:362:4)