Empty arrayList and empty string throws StringIndexOutOfBounds exception

I am getting StringIndexOutOfBounds exception in this part of the code.
The field that captures myList is optional in the UI.This error occurs only when myList/myURL string is empty. How do I handle it?

Can someone correct me what am I doing wrong here?

    if (myList != null) {

            for (int r = 0; r < myList.size(); r++) {
                myURL = myURL + myList.toString() + ",";
            }

            myURL = myURL.substring(0, myrssURL.length() - 1);
            myURL = myURL.replace("[", "").replace("]", ""); 
        }
   else
    {
        rssList.clear();
        rssURL=null;
        System.out.println("inside else >>>>");
    }