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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
v8::Local<v8::Value> val,
                     content::SavePageType* out) {
    std::string save_type;
    if (!ConvertFromV8(isolate, val, &save_type))
      return false;
    save_type = base::ToLowerASCII(save_type);
    if (save_type == "htmlonly") {
      *out = content::SAVE_PAGE_TYPE_AS_ONLY_HTML;
    } else if (save_type == "htmlcomplete") {
      *out = content::SAVE_PAGE_TYPE_AS_COMPLETE_HTML;
    } else if (save_type == "mhtml") {
      *out = content::SAVE_PAGE_TYPE_AS_MHTML;
    } else {
      return false;
    }
    return true;
  }
};
template <>
struct Converter<electron::api::WebContents::Type> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,