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 defined(OS_MACOSX)
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (view)
    view->ShowDefinitionForSelection();
#endif
}
void Ws::CopyImageAt(int x, int y) {
  auto* const host = ontents()->GetMainFrame();
  if (host)
    host->CopyImageAt(x, y);
}
void Ws::Focus() {
  ontents()->Focus();
}
#if !defined(OS_MACOSX)
bool Ws::IsFocused() const {
  auto* view = ontents()->GetRenderWidgetHostView();
  if (!view)
    return false;
  if (GetType() != Type::BACKGROUND_PAGE) {
    auto* window = ontents()->GetNativeView()->GetToplevelWindow();
    if (window && !window->IsVisible())
      return false;
  }
  return view->HasFocus();
}
#endif
void Ws::TabTraverse(bool reverse) {
  ontents()->FocusThroughTabTraversal(reverse);
}