#vulnerable code 
                                static String unescape(String string) {
        if (!string.contains("&"))
            return string;

        Matcher m = unescapePattern.matcher(string); // &(#(x|X)?([0-9a-fA-F]+)|[a-zA-Z]+);?
        StringBuffer accum = new StringBuffer(string.length()); // pity matcher can't use stringbuilder, avoid syncs

        while (m.find()) {
            int charval = -1;
            String num = m.group(3);
            if (num != null) {
                try {
                    int base = m.group(2) != null ? 16 : 10; // 2 is hex indicator
                    charval = Integer.valueOf(num, base);
                } catch (NumberFormatException e) {
                } // skip
            } else {
                String name = m.group(1).toLowerCase();
                if (full.containsKey(name))
                    charval = full.get(name);
            }

            if (charval != -1 || charval > 0xFFFF) { // out of range
                String c = Character.toString((char) charval);
                m.appendReplacement(accum, c);
            } else {
                m.appendReplacement(accum, m.group(0)); // replace with original string
            }
        }
        m.appendTail(accum);
        return accum.toString();
    } 
                            #location 18 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                static String unescape(String string) {
        if (!string.contains("&"))
            return string;

        Matcher m = unescapePattern.matcher(string); // &(#(x|X)?([0-9a-fA-F]+)|[a-zA-Z]+);?
        StringBuffer accum = new StringBuffer(string.length()); // pity matcher can't use stringbuilder, avoid syncs

        while (m.find()) {
            int charval = -1;
            String num = m.group(3);
            if (num != null) {
                try {
                    int base = m.group(2) != null ? 16 : 10; // 2 is hex indicator
                    charval = Integer.valueOf(num, base);
                } catch (NumberFormatException e) {
                } // skip
            } else {
                String name = m.group(1);
                if (full.containsKey(name))
                    charval = full.get(name);
            }

            if (charval != -1 || charval > 0xFFFF) { // out of range
                String c = Character.toString((char) charval);
                m.appendReplacement(accum, c);
            } else {
                m.appendReplacement(accum, m.group(0)); // replace with original string
            }
        }
        m.appendTail(accum);
        return accum.toString();
    }