I found a bug.
If there is only one item. selectedCls is not applied.
It seems that bottomItemRendered is 0, so getNode returns null.
I changed source a little bit like below and it is solved.

isItemRendered: function(index) {
// Trivial check after first render
if (this.store.getCount() == 1) return true; // modified. because if count is 1, bottomItemRendered is 0.
return this.bottomItemRendered > 0 ?
index >= this.topItemRendered && index <= this.bottomItemRendered : false;
},

I'm not sure this fix is safe though...