How to implement side opening child sections of RecyclerView?

Question

Java

I tried to make two parallel recyclerviews, but when you click on an element of the first list, the application crashes with the following error:

2019-08-13 12:50:42.885 8085-8085/ru.retail.app E/AndroidRuntime: FATAL EXCEPTION: main
    Process: ru.retail.app, PID: 8085
    java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference
        at ru.retail.app.Activity.ProductsActivity.readFileSD(ProductsActivity.java:146)
        at ru.retail.app.adapter.ProductsActivity_CategoriesRecyclerViewAdapter.openCategory(ProductsActivity_CategoriesRecyclerViewAdapter.java:64)
        at ru.retail.app.adapter.ProductsActivity_CategoriesRecyclerViewAdapter.access$600(ProductsActivity_CategoriesRecyclerViewAdapter.java:21)
        at ru.retail.app.adapter.ProductsActivity_CategoriesRecyclerViewAdapter$categoriesButtonListener.onClick(ProductsActivity_CategoriesRecyclerViewAdapter.java:146)
        at android.view.View.performClick(View.java:5675)
        at android.view.View$PerformClick.run(View.java:22641)
        at android.os.Handler.handleCallback(Handler.java:836)
        at android.os.Handler.dispatchMessage(Handler.java:103)
        at android.os.Looper.loop(Looper.java:203)
        at android.app.ActivityThread.main(ActivityThread.java:6251)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)

although, if both recyclerviews display the same list list, then everything is ok. Surely it can be implemented through one recyclerview, but I have no idea how to do it from the side. If anyone has come across this issue, please give me some ideas.