boot.js: _st() returns nil if argument is null

js/boot.js

@@ -792,6 +792,7 @@ var smalltalk = new Smalltalk();
 
 var _st = function(o) {
 	if(typeof o === 'undefined') {return nil}
+	if(null === o) {return nil}
 	if(o.klass) {return o}
 	return smalltalk.JSObjectProxy._on_(o);
 };