The following scripts result in crash:

var cl = new java.lang.ClassLoader(null); // -> NPE
var cl = new java.lang.ClassLoader(undefined); //-> ClassCastException
var r = new java.lang.Runnable(null); // RuntimeException that tells can't find correct constructor!
var r = new java.lang.Runnable(undefined); // same as above.

It'd be better if we uniformly throw TypeError for these cases - that tells the passed object is not a script "Object".