Changing the following line of the ui.slider.js focus function apparently solves the problem:
From: if(hard) this.currentHandle.parent().focus();
To: if(hard && this.currentHandle.parent()) this.currentHandle.parent().focus();
I have the feeling that the slider functionality mostly been tested using single-handle slider scenarios (which is probably what the slider will be used for 90% of the time). As a result, some functionality breaks when two handles and a range are present.