When developing on Kotlin, you should add the following lines next to their annotationProcessor counterparts:
kapt 'com.google.dagger:dagger-android-processor:2.15'
kapt 'com.google.dagger:dagger-compiler:2.15'
and add apply plugin: 'kotlin-kapt' at the start of the same file.
That section looks like this for me:
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt' // <- Add this line
apply plugin: 'io.fabric'