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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
content::RenderFrameHost* render_frame_host,
    const std::string& interface_name,
    mojo::ScopedMessagePipeHandle* interface_pipe) {
  registry_.TryBindInterface(interface_name, interface_pipe, render_frame_host);
}
void WebContents::DidAcquireFullscreen(content::RenderFrameHost* rfh) {
  set_fullscreen_frame(rfh);
}
void WebContents::DOMContentLoaded(
    content::RenderFrameHost* render_frame_host) {
  if (!render_frame_host->GetParent())
    Emit("dom-ready");
}
void WebContents::DidFinishLoad(content::RenderFrameHost* render_frame_host,
                                const GURL& validated_url) {
  bool is_main_frame = !render_frame_host->GetParent();
  int frame_process_id = render_frame_host->GetProcess()->GetID();
  int frame_routing_id = render_frame_host->GetRoutingID();
  auto weak_this = GetWeakPtr();
  Emit("did-frame-finish-load", is_main_frame, frame_process_id,
       frame_routing_id);
   ⚠️WARNING!⚠️
   Emit() triggers JS which can call destroy() on |this|. It's not safe to
   assume that |this| points to valid memory at this point.
  if (is_main_frame && weak_this)
    Emit("did-finish-load");
}
void WebContents::DidFailLoad(content::RenderFrameHost* render_frame_host,