InstanceProvider.kt

// interface
interface IActivityInstanceProvider {
    fun updateInstance(activity: Activity)
    fun <TActivity: Activity> get(): TActivity?
}

// class
class ActivityInstanceProvider: IActivityInstanceProvider {

    private val activityInstance: HashSet<WeakReference<Activity?>> = hashSetOf()

    override fun updateInstance(activity: Activity) {
        activityInstance.add(WeakReference(activity))
    }

    override fun <TActivity: Activity> get(): TActivity? {
        val ref = activityInstance.firstOrNull() { it.javaClass == TActivity::class }
    }
}

https://gist.github.com/mr5z/a3ed946d869cb86615cbbe71e114723c#file-instanceprovider-kt-L9
This suck ass