I didn't look into that issue that much myself yet, but I use
CustomWidgets to communicate with Applets. Which works fine with RAP 1.2.
If I run it against RAP 1.3 HEAD with all dependencies adjusted adjusted,
i get a fancy javascript error. As I said, I didn't look into that yet,
but I probably will in a few days. But I think this is stronger related to
qooxdoo than to RAP.