Fix WebContentsView crash on app quit

[kethinov]

If you do something like this in your Electron app:

const someWebView = new WebContentsView()
const ContextMenu = await import('electron-context-menu')
const contextMenu = ContextMenu.default
contextMenu({ window: someWebView })

The app will crash when you quit with an error like:

A JavaScript error occurred in the main process

Uncaught Exception:
TypeError: win.isDestroyed is not a function
at node_modules/electron-context-menu/index.js:353:11
at WebContents.disposable (node_modules/electron-context-menu/index.js:377:4)
at Object.onceWrapper (node:events:633:26)
at WebContents.emit (node:events:518:28)

This PR fixes that error.