In JavapTask, if one uses the constructors that take a Writer object, they call getPrintWriterforWriter() on the provided value:

private static PrintWriter getPrintWriterForWriter(Writer w) {
    if (w == null)
        return getPrintWriterForStream(null);
...

private static PrintWriter getPrintWriterForStream(OutputStream s) {
    return new PrintWriter(s, true);
}

if the Writer is null, then null is passed to PrintWriter, OutputStreamWriter, and then Writer. Unfortunately, Writer considers this parameter a lock object which cannot be null:

/**
 * Creates a new character-stream writer whose critical sections will
 * synchronize on the given object.
 *
 * @param lock
 * Object to synchronize on
 */
protected Writer(Object lock) {
    if (lock == null) {
        throw new NullPointerException();
    }
...

and then throws the exception.

Something other than null should be passed in this case.