AllModulesInitializer.kt

private val initializedModules = HashSet<Class<out ModuleInitializer>>()

    fun getInitializerCompletable(): Completable {
        val completableList = mutableListOf<Completable>()
        map.forEach { (clazz, moduleInitializer) ->
            completableList.addAll(getCompletableForModule(clazz, moduleInitializer))
        }

        return Completable.concat(completableList)
    }

    private fun getCompletableForModule(
        clazz: Class<out ModuleInitializer>,
        moduleToInitialize: ModuleInitializer
    ): List<Completable> {
        if (moduleToInitialize.dependencies().isEmpty() && initializedModules.contains(clazz)) {
            return emptyList()
        } else {
            val completableList = mutableListOf<Completable>()
            moduleToInitialize.dependencies().forEach {
                completableList.addAll(getCompletableForModule(it, map.getValue(it)))
            }
            if (!initializedModules.contains(clazz)) {
                initializedModules.add(clazz)
                completableList.add(moduleToInitialize.initialize(context))
            }
            return completableList
        }
    }

map[it]!! could be replaced with map.getValue(it) if you want to avoid using !!. :) Also, you get a more detailed exception instead of an NPE this way.

updated :)