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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
return ontents()->GetMainFrame()->GetProcess()->GetID();
}
base::ProcessId WebContents::GetOSProcessID() const {
  base::ProcessHandle process_handle =
      ontents()->GetMainFrame()->GetProcess()->GetProcess().Handle();
  return base::GetProcId(process_handle);
}
WebContents::Type WebContents::GetType() const {
  return type_;
}
bool WebContents::Equal(const WebContents* ontents) const {
  return ID() == ontents->ID();
}
void WebContents::LoadURL(const GURL& url,
                          const gin_helper::Dictionary& options) {
  if (!url.is_valid() || url.spec().size() > url::kMaxURLChars) {
    Emit("did-fail-load", static_cast<int>(net::ERR_INVALID_URL),
         net::ErrorToShortString(net::ERR_INVALID_URL),
         url.possibly_invalid_spec(), true);
    return;
  }
  content::NavigationController::LoadURLParams params(url);
  if (!options.Get("httpReferrer", &params.referrer)) {