fix #571 "this.__delegateMapping is null" when unloading a page
fix #571 "this.__delegateMapping is null" when unloading a page

The exception "this.__delegateMapping is null" could happen in some
cases when unloading a page. This commit fixes this small issue.

fcapa/aria,simonarbuckle/ariatemplates,fcapa/aria,ymeine/ariatemplates,divdavem/ariatemplates,fbasso/ariatemplates,ariatemplates/ariatemplates,vcarle/ariatemplates,ymeine/ariatemplates,vcarle/ariatemplates,vcarle/ariatemplates,simonarbuckle/ariatemplates,lsimone/ariatemplates,divdavem/ariatemplates,ymeine/ariatemplates,lsimone/ariatemplates,benouat/ariatemplates,fbasso/ariatemplates,divdavem/ariatemplates,simonarbuckle/ariatemplates,ariatemplates/ariatemplates,fcapa/aria,fbasso/ariatemplates,flongo/ariatemplates,mlaval/ariatemplates,benouat/ariatemplates,mlaval/ariatemplates,mlaval/ariatemplates,ariatemplates/ariatemplates,benouat/ariatemplates