* 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
window_->CloseFilePreview();
}
v8::Local<v8::Value> TopLevelWindow::GetContentView() const {
  if (content_view_.IsEmpty())
    return v8::Null(isolate());
  else
    return v8::Local<v8::Value>::New(isolate(), content_view_);
}
v8::Local<v8::Value> TopLevelWindow::GetParentWindow() const {
  if (parent_window_.IsEmpty())
    return v8::Null(isolate());
  else
    return v8::Local<v8::Value>::New(isolate(), parent_window_);
}
std::vector<v8::Local<v8::Object>> TopLevelWindow::GetChildWindows() const {
  return child_windows_.Values(isolate());
}
v8::Local<v8::Value> TopLevelWindow::GetBrowserView() const {
  if (browser_view_.IsEmpty()) {
    return v8::Null(isolate());
  }
  return v8::Local<v8::Value>::New(isolate(), browser_view_);
}
bool TopLevelWindow::IsModal() const {