Concurrent Modification Exception When Modifying ArrayList of ArrayList

Problem

Write a function that takes in an array of unique integers and returns its powerset. The powerset P(X) of a set X is the set of all subsets of X. For example, the powerset of [1,2] is [[], [1], [2], [1,2]]. Note that the sets in the powerset do not need to be in any particular order.

My approach

My approach is quite simple i will start off with a ArrayList of ArrayList called master . I will create a emptyList and add it to the master. Then i will iterate through each number and for each number i will create a a new list like all the lists in the master already but append the new number to it. so if i have a empty list inside master list and my num is 1 i will add [1] to the master. Then when i am at 2 i will add [2] and [1,2] to the master list.

MY CODE

public static void main(String args[]) {

    ArrayList<Integer>  inputList = new ArrayList<>();
    inputList.add(1);
    inputList.add(2);
    inputList.add(3);

    System.out.println(powerset(inputList).size());

}

public static ArrayList<ArrayList<Integer>> powerset(ArrayList<Integer> array) {

    ArrayList<ArrayList<Integer>> master = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> emptyList = new ArrayList<>();
    master.add(emptyList);

    for(Integer num: array){
        for(ArrayList<Integer> list: master){
            ArrayList<Integer> toAppendList = list;
            toAppendList.add(num);
            master.add(toAppendList);
        }
    }
    return master;
}

ISSUE

For some reason i keep getting

Exception in thread "main" java.util.ConcurrentModificationException

I am not sure how this is a concurrent modification and how can i remove it.