* Output of `node_modules/.bin/electron --version`:
v3.0.6
v4.0.0-beta.5

* Operating System (Platform and Version):
macOS High Siearra 10.13.6

* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable):
v2.0.11

**Expected Behavior**
BrowserWindow.getNativeWindowHandle() must return non-nullable result.

**Actual behavior**
BrowserWindow.getNativeWindowHandle() returns null.

**To Reproduce**

***Sample code***
```
const { app, BrowserWindow } = require('electron');

let win

app
	.on('ready', () => {
		win = new BrowserWindow({width: 800, height: 600})
		win.loadURL('https://www.google.com')
		
		win.on('closed', () => win = null);

		const handle = win.getNativeWindowHandle();
		console.log('Current window: ' + handle.toString('hex'));
	})
	.on('window-all-closed', () => app.quit());
```

***Expected output***
Something different than `Current window: 0000000000000000`

***Actual output***
`Current window: 0000000000000000`
atom/browser/api/atom_api_top_level_window.cc
return window_->is_modal();
}
bool TopLevelWindow::SetThumbarButtons(mate::Arguments* args) {
#if defined(OS_WIN)
  std::vector<TaskbarHost::ThumbarButton> buttons;
  if (!args->GetNext(&buttons)) {
    args->ThrowError();
    return false;
  }
  auto* window = static_cast<NativeWindowViews*>(window_.get());
  return window->taskbar_host().SetThumbarButtons(
      window_->GetAcceleratedWidget(), buttons);
#else
  return false;
#endif
}
#if defined(TOOLKIT_VIEWS)
void TopLevelWindow::SetIcon(mate::Handle<NativeImage> icon) {
#if defined(OS_WIN)
  static_cast<NativeWindowViews*>(window_.get())
      ->SetIcon(icon->GetHICON(GetSystemMetrics(SM_CXSMICON)),
                icon->GetHICON(GetSystemMetrics(SM_CXICON)));
#elif defined(USE_X11)
  static_cast<NativeWindowViews*>(window_.get())
      ->SetIcon(icon->image().AsImageSkia());
#endif
}
#endif
#if defined(OS_WIN)
bool TopLevelWindow::HookWindowMessage(UINT message,