MainActivity.kt

private const val LIST_CONTENT = ""

class MainActivity : AppCompatActivity() {
    override fun onSaveInstanceState(outState: Bundle, outPersistentState: PersistableBundle) {
        super.onSaveInstanceState(outState, outPersistentState)

        outState.putString(LIST_CONTENT,shoppingList.text.toString())
    }

    override fun onRestoreInstanceState(savedInstanceState: Bundle?, persistentState: PersistableBundle?) {
        super.onRestoreInstanceState(savedInstanceState, persistentState)

        shoppingList.text = savedInstanceState?.getString(LIST_CONTENT,"")
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val shoppingInput: EditText = findViewById(R.id.shoppingInput)
        val addBtn: Button = findViewById(R.id.addBtn)
        val shoppingList: TextView = findViewById(R.id.shoppingList)

        addBtn.setOnClickListener {
            shoppingList.append(shoppingInput.text)
            shoppingList.append("\n")
            shoppingInput.setText("")
        }
    }
}