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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
std::unique_ptr<base::Value> parsed_excluded_folders =
      base::JSONReader::ReadDeprecated(excluded_folders_message);
  if (parsed_excluded_folders && parsed_excluded_folders->is_list()) {
    for (const base::Value& folder_path : parsed_excluded_folders->GetList()) {
      if (folder_path.is_string())
        excluded_folders.push_back(folder_path.GetString());
    }
  }
  devtools_indexing_jobs_[request_id] =
      scoped_refptr<DevToolsFileSystemIndexer::FileSystemIndexingJob>(
          devtools_file_system_indexer_->IndexPath(
              file_system_path, excluded_folders,
              base::BindRepeating(
                  &WebContents::OnDevToolsIndexingWorkCalculated,
                  weak_factory_.GetWeakPtr(), request_id, file_system_path),
              base::BindRepeating(&WebContents::OnDevToolsIndexingWorked,
                                  weak_factory_.GetWeakPtr(), request_id,
                                  file_system_path),
              base::BindRepeating(&WebContents::OnDevToolsIndexingDone,
                                  weak_factory_.GetWeakPtr(), request_id,
                                  file_system_path)));
}
void WebContents::DevToolsStopIndexing(int request_id) {
  auto it = devtools_indexing_jobs_.find(request_id);
  if (it == devtools_indexing_jobs_.end())
    return;
  it->second->Stop();
  devtools_indexing_jobs_.erase(it);
}
void WebContents::DevToolsSearchInPath(int request_id,