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### Issue Details
webContents.fromId() for non existing id (e.g. removed webview) causes segmentation fault in Electron 11.
In Electron 9 & 10 null was returned.
Same problem on Windows and macOS Catalina 

* **Electron Version:**
  * 11.0.1
* **Operating System:**
  * macOS 11.0.1
* **Last Known Working Electron version:**
  * 10.1.6

### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->
webContents.fromId() returns null for non existing id

### Actual Behavior
<!-- A clear and concise description of what actually happened. -->
Segmentation fault

### To Reproduce
https://gist.github.com/larssvensby/071353ec109c4975fd28b5a4b1dc7ad5

### Screenshots
<!-- If applicable, add screenshots to help explain your problem. -->

### Additional Information
<!-- Add any other context about the problem here. -->

shell/browser/api/electron_api_web_contents.cc
mojo::PendingReceiver<mojom::ElectronBrowser> receiver,
    content::RenderFrameHost* render_frame_host) {
  auto id = receivers_.Add(this, std::move(receiver), render_frame_host);
  frame_to_receivers_map_[render_frame_host].push_back(id);
}
void WebContents::OnElectronBrowserConnectionError() {
  auto receiver_id = receivers_.current_receiver();
  auto* frame_host = receivers_.current_context();
  base::Erase(frame_to_receivers_map_[frame_host], receiver_id);
}
void WebContents::Message(bool internal,
                          const std::string& channel,
                          blink::CloneableMessage arguments) {
  TRACE_EVENT1("electron", "WebContents::Message", "channel", channel);
   webContents.emit('-ipc-message', new Event(), internal, channel,
   arguments);
  EmitWithSender("-ipc-message", receivers_.current_context(), InvokeCallback(),
                 internal, channel, std::move(arguments));
}
void WebContents::Invoke(bool internal,