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
Emit("will-destroy");
  ResetManagedWs(async);
}
bool Ws::DidAddMessageToConsole(
    content::Ws* source,
    blink::mojom::ConsoleMessageLevel level,
    const base::string16& message,
    int32_t line_no,
    const base::string16& source_id) {
  return Emit("console-message", static_cast<int32_t>(level), message, line_no,
              source_id);
}
void Ws::OnCreateWindow(