[Mac] Fix loading local resources on Webview

[Miepee]

This just used the info from #2380, I don't currently have access to a Mac, so I didn't test this.

@somelinguist can you (or someone else) use the action-built test suite from here to see whether the fix correctly works?

Fixes #2380 (hopefully)

[cwensley]

Hey @Miepee, thanks for the fix! This is great, however I think you are missing a null check on baseUri before using it, which is its default value.

[Miepee]

As far as I can see, the ToNS() extension method already includes the null check.

        public static NSUrl ToNS(this Uri uri)
        {
            return uri == null ? null : new NSUrl(uri.AbsoluteUri);
        }

[cwensley]

While that is true, the WKWebView.LoadFileUrl() call does not allow null for either of its parameters, so it will crash.