class MyGenericCustomView<T>(context: Context, attrs: AttributeSet) : AnotherView(context, attrs) {
    ...
}

<package.name.MyGenericCustomView
   android:id="@+id/custom_id"
   ....
/>

override fun onCreate(...) {
    ...
    val myCustomView = findViewById<MyGenericCustomView<String>>(R.id.custom_id)
    ...
}

//custom_id is of type MyGenericCustomView<*>

class MySpecificCustomView(context: Context, attrs: AttributeSet) : MyGenericCustomView<String>(context, attrs) {
    ....
}

val myView = custom_id as MyGenericCustomView<String>

class MyActivity() : Activity(…) {

    val myView by lazy { custom_id as MyGenericCustomView<String> }

    ...
}