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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* context = ontents->GetBrowserContext();
  return static_cast<electron::ElectronBrowserContext*>(context)->prefs();
}
std::map<std::string, std::string> GetAddedFileSystemPaths(
    content::WebContents* ontents) {
  auto* pref_service = GetPrefService(ontents);
  const base::DictionaryValue* file_system_paths_value =
      pref_service->GetDictionary(prefs::kDevToolsFileSystemPaths);
  std::map<std::string, std::string> result;
  if (file_system_paths_value) {
    base::DictionaryValue::Iterator it(*file_system_paths_value);
    for (; !it.IsAtEnd(); it.Advance()) {
      std::string type =
          it.value().is_string() ? it.value().GetString() : std::string();
      result[it.key()] = type;
    }
  }
  return result;
}
bool IsDevToolsFileSystemAdded(content::WebContents* ontents,
                               const std::string& file_system_path) {
  auto file_system_paths = GetAddedFileSystemPaths(ontents);
  return file_system_paths.find(file_system_path) != file_system_paths.end();
}
e
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
WebContents::Type GetTypeFromViewType(extensions::ViewType view_type) {