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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
}
 static
gin::Wo WebContents::kWo = {gin::kEmbedderNativeGin};
e api
e electron
namespace {
using electron::api::GetAllWebContents;
using electron::api::WebContents;
std::vector<gin::HContents>> GetAllWebContentsAsV8(
    v8::Isolate* isolate) {
  std::vector<gin::HContents>> list;
  for (auto iter = base::IDMap<WebContents*>::iterator(&GetAllWebContents());
       !iter.IsAtEnd(); iter.Advance()) {
    list.push_back(gin::CreateHandle(isolate, iter.GetCurrentValue()));
  }
  return list;
}
void Initialize(v8::Lobject> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  gin_helper::Dictionary dict(isolate, exports);
  dict.Set("WebContents", WebContents::GetConstructor(context));
  dict.SetMethod("fromId", &WebContents::FromID);
  dict.SetMethod("getAllWebContents", &GetAllWebContentsAsV8);
}
e
NODULE_CONTEXT_AWARE(electron_browser_ontents, Initialize)