Q: Fabric Beta release notes and Gradle Kotlin DSL As stated here: https://docs.fabric.io/android/beta/gradle.html
I used to have this 2 lines in an Android app's build.gradle to upload builds to Fabric Beta with given release notes to given group aliases:
android {
    defaultConfig {
        ext.betaDistributionReleaseNotesFilePath = 'app/release_notes.txt'
        ext.betaDistributionGroupAliasesFilePath = 'app/group_aliases.txt'
    }
}

After migrating the build.gradle files to Kotlin Gradle DSL these 2 lines have become:
android {
    defaultConfig {
        ext.set("betaDistributionReleaseNotesFilePath", "app/release_notes.txt")
        ext.set("betaDistributionGroupAliasesFilePath", "app/group_aliases.txt")
    }
}

But they no longer work: all builds published to Fabric Beta are with blank release notes and no group aliases.
I've tried several alternative syntaxes to set the extra properties:
val betaDistributionReleaseNotesFilePath by extra { "app/release_notes.txt" }

or 
ext["betaDistributionReleaseNotesFilePath"] = "app/release_notes.txt")

or
extra.set("betaDistributionReleaseNotesFilePath", "app/release_notes.txt")

or
project.ext.set("betaDistributionReleaseNotesFilePath", "app/release_notes.txt")

But results don't change.
Adding and running a debug task defined as:
tasks.create("myTask") {
    dependsOn("assembleDebug", "crashlyticsUploadDistributionDebug")

    doLast {

        val a = ext["betaDistributionReleaseNotesFilePath"]
        val b = ext["betaDistributionGroupAliasesFilePath"]

        println("ReleaseNotes: $a")
        println("GroupAliases: $b")
    }
}

Prints:
ReleaseNotes: app/release_notes.txt
GroupAliases: app/group_aliases.txt

only if the extra properties have been set using project.ext.set(), but still the uploaded builds have blank release notes and no group aliases.
What am I getting wrong?

A: you can use:
 configure<CrashlyticsExtension> {

            betaDistributionReleaseNotes = "xxx"
        }

