apps masterpage replaced by seatle.master and not working

Hi,

I installed the corporat news and add lots of data and then I chenged the seatle.master and published to all subsites.

It was published to app site for corporate too.

the error was saying the seatle can not be used for app site.

I used this lines to change it back to app.master but it's not working and totally broken.

webItem.MasterUrl = (new Uri("/_catalogs/masterpage/app.master"));
webItem.CustomMasterUrl = (new Uri("/_catalogs/masterpage/app.master"));

webItem.Update();

how should I fix the broken app.

The resource cannot be found.

thanks for any help.