Created attachment 11711 [details]
web app to reproduce; its source code and class files

How about mentioning the exception as well, in case the problem is obvious ?

The test is bad (it's an EAR): please include a regular test case, as well as
the exception report.
BTW, given the presence of the default servlet, I do not see how you can expect
to get a null RD back (yet another weird test, maybe ?).

I bet this is a duplicate of 23211

http://issues.apache.org/bugzilla/show_bug.cgi?id=23211

Created attachment 11712 [details]
Simpler version of the test case - same URL to reproduce

Exception thrown:
java.lang.NullPointerException

I can't quite see how your package is simpler.
I did waste some time investigating (while my right eye was watching roland
garros ;) ), and your bug is really invalid: the NPE is thrown somewhere in your
code, and it works (apparently) after setting crossContext in DefaultContext.
As a general rule, don't bother filing a bug if:
- we can't really know for sure what your test is doing and/or you don't want to
tell
- you can't post stack traces

Created attachment 11713 [details]
third attachment with correct path to a servlet

To respond to Tim Funk's comment -- created a third test case attachment so that
a correct servlet path is used when place call
ServletContext.getRequestDispatcher(path_servlet).  

To respond to Remy's comment #1 - with code like this, the exception is from
Tomcat:
            try {
                rd = context.getRequestDispatcher(path_servlet);
            } catch (Exception ex) {
                out.println("Unexpected Exception thrown:" + ex.toString());
            }

To respond to Remy's comment #2 -- "it works (apparently) after setting
crossContext in DefaultContext" -- It should also return null if crossContext
not set, not just throwing an exception.   Both of them together(!!!) make the
picture complete.

Whatever ;) I won't waste any more time of this issue, that's for sure.

If context == null (which it will be by default), then surprise surprise, 
*your* code will throw an NPE.

Please go back and read Remy's comments about how to submit a bug report.

Well, to respond to William Barker's comment - BIG surprise that you didn't read
the source code before respond - to quote the more code in the third
attachement:

        ses = req.getSession(true);
        if (ses != null) {
            ServletContext context =
getServletConfig().getServletContext().getContext(path_context);

            try {
                rd = context.getRequestDispatcher(path_servlet);
            } catch (Exception ex) {
                out.println("Unexpected Exception thrown:" + ex.toString());
            }

        } else {
            out.println("Cannot get a HttpSession");
        }


To respond to Remy's comment since I am back in here  -  I think this is
important to fix - crossContext maynot be always turned on for security reason.

If crossContext is false then "getContext(path_context)" would return null.

If context is null then trying to call one of it's methods will always yield a
NullPointerException.
 rd = context.getRequestDispatcher(path_servlet);

IOW: There is no context to respond to the "getRequestDispatcher" call.

Response to Ben Souther and william.barker, thanks, ServletContext returned Null
in this case.  

Test error.