NullSafety on extern module field

[R32]

Compile the following code will get an error: Null safety: Field "document" is not nullable thus should have an initial value.

but I guess that nullsafety should not verify external fields.

Main.hx

package;

class Main {
	static function main() {
		trace(document.URL);
	}
}

@:native("document") extern final document : js.html.Document;

build.hxml

--macro nullSafety('Main')
-main Main
--js bin/main.js