If a JSP page is called with no query string and I try to check for the existance of a particular tag with <req:existsQueryString ...> I get a NullPointerException from ExistsQueryStringTag.java:117 calling java.util.StringTokenizer. Looking at the code it doesn't check that the querystring isn't null before trying to tokenize it, it should be a trivial change to fix this.
Thanks for reporting this. Sorry it took so long for me to get to it. Fixed and will be in the next nightly build.