Issue in gradle build 5.6.4

Your original error was because the property classesDir was removed. The new property classesDirs is a collection of directories now, not a single value. The following line only takes a single value:

fileset(dir: sourceSets[sourceSetName].output.classesDir, includes: '**/*.class')

which means you’re going to need to write code to call fileset() once for each value in the collection, not just change the classesDir to classesDirs. i.e.

sourceSets.main.output.classesDirs.each { classesDir ->
    fileset(dir: classesDir, includes: '**/*.class')
}