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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
.SetMethod("_setNextChildWebPreferences",
                 &WebContents::SetNextChildWebPreferences)
      .SetMethod("addWorkSpace", &WebContents::AddWorkSpace)
      .SetMethod("removeWorkSpace", &WebContents::RemoveWorkSpace)
      .SetMethod("showDefinitionForSelection",
                 &WebContents::ShowDefinitionForSelection)
      .SetMethod("copyImageAt", &WebContents::CopyImageAt)
      .SetMethod("capturePage", &WebContents::CapturePage)
      .SetMethod("setEmbedder", &WebContents::SetEmbedder)
      .SetMethod("setDevToolsWebContents", &WebContents::SetDevToolsWebContents)
      .SetMethod("getNativeView", &WebContents::GetNativeView)
      .SetMethod("incrementCapturerCount", &WebContents::IncrementCapturerCount)
      .SetMethod("decrementCapturerCount", &WebContents::DecrementCapturerCount)
      .SetMethod("isBeingCaptured", &WebContents::IsBeingCaptured)
      .SetMethod("setWebRTCIPHandlingPolicy",
                 &WebContents::SetWebRTCIPHandlingPolicy)
      .SetMethod("getWebRTCIPHandlingPolicy",
                 &WebContents::GetWebRTCIPHandlingPolicy)
      .SetMethod("_grantOriginAccess", &WebContents::GrantOriginAccess)
      .SetMethod("takeHeapSnapshot", &WebContents::TakeHeapSnapshot)
      .SetProperty("id", &WebContents::ID)
      .SetProperty("session", &WebContents::Session)
      .SetProperty("hostWebContents", &WebContents::HostWebContents)
      .SetProperty("devToolsWebContents", &WebContents::DevToolsWebContents)
      .SetProperty("debugger", &WebContents::Debugger)
      .SetProperty("_initiallyShown", &WebContents::WasInitiallyShown)
      .SetProperty("mainFrame", &WebContents::MainFrame)
      .Build();
}
const char* WebContents::GetTypeName() {