Unity API: Throw exceptions when IWebView is called while not ready (causes crash!)

[ArDevKarl]

I'm using this to display webpages on the HoloLens:
https://learn.microsoft.com/en-us/windows/mixed-reality/develop/advanced-concepts/webview2-unity-plugin

Unity or the application crashes without notice when calling IWebView while not ready. Please, make sure to catch this crashing error and throw an Exception, if this is the case.

Together with this #3722, the WebView would be much easier to implement for general purposes.

AB#46857724