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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
blink::CloneableMessage arguments) {
  TRACE_EVENT1("electron", "WebContents::MessageHost", "channel", channel);
   webContents.emit('ipc-message-host', new Event(), channel, args);
  EmitWithSender("ipc-message-host", receivers_.current_context(),
                 InvokeCallback(), channel, std::move(arguments));
}
void WebContents::UpdateDraggableRegions(
    std::vector<mojom::DraggableRegionPtr> regions) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnDraggableRegionsUpdated(regions);
}
void WebContents::RenderFrameDeleted(
    content::RenderFrameHost* render_frame_host) {
   A WebFrameMain can outlive its RenderFrameHost so we need to mark it as
   disposed to prevent access to it.
  WebFrameMain::RenderFrameDeleted(render_frame_host);
   A RenderFrameHost can be destroyed before the related Mojo binding is
   closed, which can result in Mojo calls being sent for RenderFrameHosts
   that no longer exist. To prevent this from happening, when a
   RenderFrameHost goes away, we close all the bindings related to that
   frame.
  auto it = frame_to_receivers_map_.find(render_frame_host);
  if (it == frame_to_receivers_map_.end())
    return;
  for (auto id : it->second)
    receivers_.Remove(id);
  frame_to_receivers_map_.erase(it);
}
void WebContents::DidStartNavigation(