Null object reference error in java with char arrays

Question

I have a char[] that I am attempting to append values to. However, at the line newStringAr[count] = ar[j][i], I am receiving an error: java.lang.NullPointerException: Attempt to invoke virtual method 'char java.lang.Character.charValue()' on a null object reference.

When I changed my array to a Character[], this error was resolved, but then I was not able to convert my array to a string as shown in the last line of the code sample below. How can I resolve this issue?

Note that ar is a previously-defined array of Characters (Character[] not char[]).

    int count = 0;
    char[] newStringAr = new char[ch.length];

    for (int i = 0; i < ar.length; i++) {
        for (int j = 0; j < ar.length; j++) {
            if (count<ch.length) {
                newStringAr[count] = ar[j][i];
                count++;
            }
        }
    }

    String out = new String(newStringAr);