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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* web_preferences = WebContentsPreferences::From(ontents());
  DCHECK(web_preferences);
  web_preferences->preference()->SetKey("ignoreMenuShortcuts",
                                        base::Value(ignore));
}
void WebContents::SetAudioMuted(bool muted) {
  ontents()->SetAudioMuted(muted);
}
bool WebContents::IsAudioMuted() {
  return ontents()->IsAudioMuted();
}
bool WebContents::IsCurrentlyAudible() {
  return ontents()->IsCurrentlyAudible();
}
#if BUILDFLAG(ENABLE_PRINTING)
void WebContents::OnGetDefaultPrinter(
    base::Value print_settings,
    printing::CompletionCallback print_callback,
    base::string16 device_name,
    bool silent,
    base::string16 default_printer) {
   The content::WebContents might be already deleted at this point, and the