Reject add if permission selector has not been set

src/com/gitblit/wicket/panels/RegistrantPermissionsPanel.java

 			protected void onSubmit(AjaxRequestTarget target, Form<?> form) {
 				// add permission to our list
 				RegistrantAccessPermission rp = (RegistrantAccessPermission) form.getModel().getObject();
+				if (rp.permission == null) {
+					return;
+				}
 				RegistrantAccessPermission copy = DeepCopier.copy(rp);
 				if (StringUtils.findInvalidCharacter(copy.registrant) != null) {
 					copy.permissionType = PermissionType.REGEX;