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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
const base::FilePath& path) {
  auto* isolated_context = storage::IsolatedContext::GetInstance();
  std::string root_name(kRootName);
  storage::IsolatedContext::ScopedFSHandle file_system =
      isolated_context->RegisterFileSystemForPath(
          storage::kFileSystemTypeNativeLocal, std::string(), path, &root_name);
  content::ChildProcessSecurityPolicy* policy =
      content::ChildProcessSecurityPolicy::GetInstance();
  content::RenderViewHost* render_view_host = ontents->GetRenderViewHost();
  int renderer_id = render_view_host->GetProcess()->GetID();
  policy->GrantReadFileSystem(renderer_id, file_system.id());
  policy->GrantWriteFileSystem(renderer_id, file_system.id());
  policy->GrantCreateFileForFileSystem(renderer_id, file_system.id());
  policy->GrantDeleteFromFileSystem(renderer_id, file_system.id());
  if (!policy->CanReadFile(renderer_id, path))
    policy->GrantReadFile(renderer_id, path);
  return file_system.id();
}
FileSystem CreateFileSystemStruct(content::WebContents* ontents,
                                  const std::string& file_system_id,
                                  const std::string& file_system_path,
                                  const std::string& type) {
  const GURL origin = ontents->GetURL().GetOrigin();
  std::string file_system_name =
      storage::GetIsolatedFileSystemName(origin, file_system_id);
  std::string root_url = storage::GetIsolatedFileSystemRootURIString(
      origin, file_system_id, kRootName);
  return FileSystem(type, file_system_name, root_url, file_system_path);
}
std::unique_ptr<base::DictionaryValue> CreateFileSystemValue(