Attempt to use the "-helpfile" option leads to the following error:
javadoc: error - Option reused: -helpfile

The reason is the inappropriate value checking for null after correct initialization of the "helpfile" variable (jdk.javadoc.internal.doclets.formats.html.ConfigurationImpl.validateOptions()):

..............
    protected boolean validateOptions() {
..............
        if (opt.matches("-helpfile")) {
..............
            if (this.helpfile != null) {
                reporter.print(ERROR, getText("doclet.Option_reuse",
                    "-helpfile"));
                return false;
            }