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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 "electron/buildflags/buildflags.h"
#include "electron/shell/common/api/api.mojom.h"
#include "gin/arguments.h"
#include "gin/data_object_builder.h"
#include "gin/handle.h"
#include "gin/object_template_builder.h"
#include "gin/wrappable.h"
#include "mojo/public/cpp/bindings/associated_remote.h"
#include "mojo/public/cpp/bindings/pending_receiver.h"
#include "mojo/public/cpp/bindings/remote.h"
#include "mojo/public/cpp/system/platform_handle.h"
#include "ppapi/buildflags/buildflags.h"
#include "printing/buildflags/buildflags.h"
#include "shell/browser/api/electron_api_browser_window.h"
#include "shell/browser/api/electron_api_debugger.h"
#include "shell/browser/api/electron_api_session.h"
#include "shell/browser/api/electron_api_web_frame_main.h"
#include "shell/browser/api/message_port.h"
#include "shell/browser/browser.h"
#include "shell/browser/child_ontents_tracker.h"
#include "shell/browser/electron_autofill_driver_factory.h"
#include "shell/browser/electron_browser_client.h"
#include "shell/browser/electron_browser_context.h"
#include "shell/browser/electron_browser_main_parts.h"
#include "shell/browser/electron_javascript_dialog_manager.h"
#include "shell/browser/electron_navigation_throttle.h"
#include "shell/browser/lib/bluetooth_chooser.h"
#include "shell/browser/native_window.h"
#include "shell/browser/session_preferences.h"
#include "shell/browser/ui/drag_util.h"