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("did-stop-loading");
}
bool Ws::EmitNavigationEvent(
    const std::string& event,
    content::NavigationHandle* navigation_handle) {
  bool is_main_frame = navigation_handle->IsInMainFrame();
  int frame_tree_node_id = navigation_handle->GetFrameTreeNodeId();
  content::FrameTreeNode* frame_tree_node =
      content::FrameTreeNode::GloballyFindByID(frame_tree_node_id);
  content::RenderFrameHostManager* render_manager =
      frame_tree_node->render_manager();
  content::RenderFrameHost* frame_host = nullptr;
  if (render_manager) {
    frame_host = render_manager->speculative_frame_host();
    if (!frame_host)
      frame_host = render_manager->current_frame_host();
  }
  int frame_process_id = -1, frame_routing_id = -1;
  if (frame_host) {
    frame_process_id = frame_host->GetProcess()->GetID();
    frame_routing_id = frame_host->GetRoutingID();
  }
  bool is_same_document = navigation_handle->IsSameDocument();
  auto url = navigation_handle->GetURL();
  return Emit(event, url, is_same_document, is_main_frame, frame_process_id,
              frame_routing_id);
}
void Ws::BindElectronBrowser(