Problem when build signed apk

[IndiceDev]

When I try to generate signed apk I get some erros. But when I build a debug aplication, it works.
I don't know if I'm doing something wrong or I forgot to set up something.

Warning:com.kosalgeek.genasync12.MainActivity: can't find referenced class com.kosalgeek.genasync12.R$layout

Warning:com.kosalgeek.genasync12.MainActivity: can't find referenced class com.kosalgeek.genasync12.R$id

Warning:com.kosalgeek.genasync12.MainActivity: can't find referenced class com.kosalgeek.genasync12.R$layout

Warning:com.kosalgeek.genasync12.MainActivity: can't find referenced class com.kosalgeek.genasync12.R$id

Warning:com.kosalgeek.genasync12.MainActivity: can't find referenced class com.kosalgeek.genasync12.R

Error:Execution failed for task ':app:transformClassesAndResourcesWithProguardForRelease'.

Job failed, see logs for details