NullPointerException fix when calling JS methods exposed as JavaProxys - JSR223

[EduFrazao]

With a JavaScript like that

    function run() {
         print('Some string');
    }

If I try to invoke it from a JavaProxy using this code, I got a NullPointerException

   final ScriptEngine engine = engineManager.getEngineByName("rhino");
   engine.eval(myScript);
   Invocable inv = (Invocable) engine;
   Runnable runnable = inv.getInterface(Runnable.class);
   runnable.run();

[rbri]

@EduFrazao can you please add an unit test to your PR

[p-bakker]

Converted to draft as it currently is missing a testcase