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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
gfx::Rect rect;
  gin_helper::Promise<gfx::Image> promise(args->isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
   get rect arguments if they exist
  args->GetNext(&rect);
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (!view) {
    promise.Resolve(gfx::Image());
    return handle;
  }
   Capture full page if user doesn't specify a |rect|.
  const gfx::Size view_size =
      rect.IsEmpty() ? view->GetViewBounds().size() : rect.size();
   By default, the requested bitmap size is the view size in screen
   coordinates.  However, if there's more pixel detail available on the
   current system, increase the requested bitmap size to capture it all.
  gfx::Size bitmap_size = view_size;
  const gfx::NativeView native_view = view->GetNativeView();
  const float scale = display::Screen::GetScreen()
                          ->GetDisplayNearestView(native_view)
                          .device_scale_factor();
  if (scale > 1.0f)
    bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  view->CopyFromSurface(gfx::Rect(rect.origin(), view_size), bitmap_size,
                        base::BindOnce(&OnCapturePageDone, std::move(promise)));
  return handle;
}
void WebContents::IncrementCapturerCount(gin::Arguments* args) {