Electron Version: 7.1.1
Operating System: Windows 10 (1803)

If I set the 'margins' option when call webContents.print method,
The app will crash with no error message.

```js
let { remote } = require("electron");
let webContents = remote.getCurrentWebContents();
webContents.print(
  {
    margins: {
      marginType: "custom",
      top: 30,
      bottom: 0,
      left: 0,
      right: 0
    }
  },
  (success, errorType) => {
    if (!success) console.log(errorType);
  }
);
```

shell/browser/api/electron_api_web_contents.cc
if (managed_ontents()) {
    managed_ontents()->GetView()->SetDelegate(nullptr);
    RenderViewDeleted(ontents()->GetRenderViewHost());
    if (type_ == Type::BROWSER_WINDOW && owner_window()) {
      
      
      for (ExtendedWsObserver& observer : observers_)
        observer.OnCloseContents();
      
      
      WsDestroyed();
    } else if (Browser::Get()->is_shutting_down()) {
      
      DestroyWs(false );
    } else {
      
      
      DestroyWs(!IsGuest() );
      
      
      
      WsDestroyed();
    }
  }
}
void Ws::DestroyWs(bool async) {