Check for null before validating again regex.

Change-Id: I7ed08f0738e581840b28851d080b06657299d55b
Reviewed-on: http://gerrit.dmdirc.com/3569
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>

src/com/dmdirc/util/validators/RegexStringValidator.java

     /** {@inheritDoc} */
     @Override
     public ValidationResponse validate(final String object) {
-        if (object.matches(regex)) {
+        if (object != null && object.matches(regex)) {
             return new ValidationResponse();
         } else {
             return new ValidationResponse(failedReason);