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* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
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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
} else {
      const net::HttpResponseHeaders* http_response =
          navigation_handle->GetResponseHeaders();
      std::string http_status_text;
      int http_response_code = -1;
      if (http_response) {
        http_status_text = http_response->GetStatusText();
        http_response_code = http_response->response_code();
      }
      Emit("did-frame-navigate", url, http_response_code, http_status_text,
           is_main_frame, frame_process_id, frame_routing_id);
      if (is_main_frame) {
        Emit("did-navigate", url, http_response_code, http_status_text);
      }
    }
    if (IsGuest())
      Emit("load-commit", url, is_main_frame);
  } else {
    auto url = navigation_handle->GetURL();
    int code = navigation_handle->GetNetErrorCode();
    auto description = net::ErrorToShortString(code);
    Emit("did-fail-provisional-load", code, description, url, is_main_frame,
         frame_process_id, frame_routing_id);
     Do not emit "did-fail-load" for canceled requests.
    if (code != net::ERR_ABORTED)
      Emit("did-fail-load", code, description, url, is_main_frame,
           frame_process_id, frame_routing_id);
  }
}
void WebContents::TitleWasSet(content::NavigationEntry* entry) {