Web viewer exists only with URL loaded
To verify, here a FileMaker window in 3D view in debugger, where you see the view hierarchy without a web viewer:

And here after loading about:blank with the web viewer:

As you see FMWKWebView is now present and MBS Plugin can find it with WebView.FindByName and other functions. Alternatively you can use WebView.Create or WebView.CreateOnPopover to add a web view via plugin functions.