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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
#include "components/prefs/scoped_user_pref_update.h"
#include "components/security_state/content/content_utils.h"
#include "components/security_state/core/security_state.h"
#include "content/browser/renderer_host/frame_tree_node.h"  
#include "content/browser/renderer_host/render_frame_host_manager.h"  
#include "content/browser/renderer_host/render_widget_host_impl.h"  
#include "content/browser/renderer_host/render_widget_host_view_base.h"  
#include "content/public/browser/child_process_security_policy.h"
#include "content/public/browser/context_menu_params.h"
#include "content/public/browser/download_request_utils.h"
#include "content/public/browser/favicon_status.h"
#include "content/public/browser/file_select_listener.h"
#include "content/public/browser/native_web_keyboard_event.h"
#include "content/public/browser/navigation_details.h"
#include "content/public/browser/navigation_entry.h"
#include "content/public/browser/navigation_handle.h"
#include "content/public/browser/plugin_service.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/render_widget_host.h"
#include "content/public/browser/render_widget_host_view.h"
#include "content/public/browser/security_style_explanation.h"
#include "content/public/browser/security_style_explanations.h"
#include "content/public/browser/service_worker_context.h"
#include "content/public/browser/site_instance.h"
#include "content/public/browser/storage_partition.h"
#include "content/public/browser/ontents.h"
#include "content/public/common/referrer_type_converters.h"
#include "content/public/common/webplugininfo.h"