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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
auto* browser_context = ontents()->GetBrowserContext();
  auto* download_manager =
      content::BrowserContext::GetDownloadManager(browser_context);
  std::unique_ptr<download::DownloadUrlParameters> download_params(
      content::DownloadRequestUtils::CreateDownloadForWebContentsMainFrame(
          ontents(), url, MISSING_TRAFFIC_ANNOTATION));
  download_manager->DownloadUrl(std::move(download_params));
}
GURL WebContents::GetURL() const {
  return ontents()->GetURL();
}
base::string16 WebContents::GetTitle() const {
  return ontents()->GetTitle();
}
bool WebContents::IsLoading() const {
  return ontents()->IsLoading();
}
bool WebContents::IsLoadingMainFrame() const {
  return ontents()->IsLoadingToDifferentDocument();
}
bool WebContents::IsWaitingForResponse() const {
  return ontents()->IsWaitingForResponse();
}
void WebContents::Stop() {
  ontents()->Stop();
}
void WebContents::GoBack() {