What can I do to make the button clickable?

Question

Android

Hello, and thanks in advance for your help. I want to say right away that most of this code is not mine. I'm just learning Kotlin and that's why I downloaded a small project from Github and I'm trying to figure it out, adding something of my own.

There will be a lot of code in the question, but I tried to shorten it by cutting out the imports.

Here is my short question:
Below you can see a screenshot of the form. As you can see, there are several buttons on the screen, and all of them work according to a similar principle: they add a product or remove it, taking the product itself as an argument. Unfortunately, I don't understand how to make the "Order" button work. In fact, I just need a stub that outputs the toast "Order completed". But each button on this form must take a product as an argument. Trying to just execute a function with toast, by analogy with what is in the following code files, I get an error.

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference.

Here is the form itself:


fragment_cart.xml - form code.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".ui.cart.CartFragment"
    android:orientation="vertical"
    android:background="#000000">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginHorizontal="@dimen/_16sdp"
        android:layout_marginBottom="@dimen/_8sdp">
        <Button
            android:id="@+id/btn_order"
            android:layout_width="wrap_content"
            android:layout_height="@dimen/_48sdp"
            android:layout_alignParentBottom="true"
            android:background="@drawable/bg_button_green"
            android:clickable="true"
            android:focusable="true"
            android:text="Заказать"
            android:textAllCaps="false"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</LinearLayout>

Below I provide the code for the logic of the buttons, as well as the product model.

CartFragment.kt

class CartFragment : Fragment(R.layout.fragment_cart) {

    private val viewModel: CartViewModel by viewModel()


    private val cartAdapter: CartAdapter by lazy {
        CartAdapter(object : OnTotalChange{
            override fun onTotalChange(total: Int) {
                val dec = DecimalFormat("#,###")
                val priceRupiah = dec.format(total)

                tv_total_price.text = "$priceRupiah руб"
            }
        })
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)


        observeCart()
        setListCart()
        viewModel.loadDataCart()

    }

    private fun observeCart() {
        viewModel.cartProduct.observe(viewLifecycleOwner, {
            cartAdapter.setDataAdapter(it)
        })
    }

    private fun setListCart() {
        rv_cart.setHasFixedSize(true)
        rv_cart.adapter = cartAdapter
        cartAdapter.onClickListener = object : OnClickItemAddRemove {
            override fun onClick(productEntity: ProductEntity) {
                val intent = Intent(activity, DetailProductActivity::class.java)
                intent.putExtra(Constant.DATA, productEntity)
                startActivity(intent)
            }

            override fun onClickAdd(productEntity: ProductEntity) {
                addQtyProduct(productEntity)
            }

            override fun onClickSubstract(productEntity: ProductEntity) {
                substractQtyProduct(productEntity)
            }

            override fun onClickRemove(productEntity: ProductEntity) {
                removeFromCart(productEntity)
            }

<b>            override fun onClickOrderCreate(productEntity: ProductEntity) {
                orderCreate(productEntity)
            }</b>
            }
    }

    private fun addQtyProduct(productEntity: ProductEntity) {
        viewModel.addProduct(productEntity)
        viewModel.loadDataCart()
    }

    private fun substractQtyProduct(productEntity: ProductEntity) {
        viewModel.subtractProduct(productEntity)
        viewModel.loadDataCart()
    }

    private fun removeFromCart(productEntity: ProductEntity) {
        viewModel.removeProduct(productEntity, ProductSavedType.CART)
        Toast.makeText(activity, "Продукт удалён", Toast.LENGTH_SHORT).show()
        viewModel.loadDataCart()
    }
<b>    private fun orderCreate(productEntity: ProductEntity) {
        Toast.makeText(activity, "Заказ офомрлен", Toast.LENGTH_SHORT).show()
    }</b>
}

OnClickItem.kt

interface OnClickItem {
    fun onClick(productEntity: ProductEntity)
}

interface OnClickItemAndAdd {
    fun onClick(productEntity: ProductEntity)
    fun onClickAdd(productEntity: ProductEntity)
}

interface OnClickItemAddRemove {
    fun onClick(productEntity: ProductEntity)
    fun onClickAdd(productEntity: ProductEntity)
    fun onClickSubstract(productEntity: ProductEntity)
    fun onClickRemove(productEntity: ProductEntity)
<b>    fun onClickOrderCreate(productEntity: ProductEntity)</b>
}

interface myOnClick {


}

CartAdapter.kt

class CartAdapter(val listener : OnTotalChange): RecyclerView.Adapter<CartAdapter.CartViewHolder>() {

    private var list: MutableList<ProductEntity> = mutableListOf()
    var onClickListener: OnClickItemAddRemove? =null
    var myOnClickListener: myOnClick? =null

    inner class CartViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        @SuppressLint("SetTextI18n")
        fun bind(productEntity: ProductEntity) {

            val price = productEntity.priceToQty
            val dec = DecimalFormat("#,###")
            val priceRupiah = dec.format(price)

            itemView.setOnClickListener {
                onClickListener?.onClick(productEntity)
            }

            itemView.btn_delete_cart.setOnClickListener {
                onClickListener?.onClickRemove(productEntity)
            }

            itemView.btn_min_cart.setOnClickListener {
                onClickListener?.onClickSubstract(productEntity)
            }

            itemView.btn_plus_cart.setOnClickListener {
                onClickListener?.onClickAdd(productEntity)
            }

<b>            itemView.btn_order.setOnClickListener {
                onClickListener?.onClickOrderCreate(productEntity)
            }</b>

            Glide.with(itemView)
                .load(productEntity.picture)
                .transition(DrawableTransitionOptions.withCrossFade())
                .fitCenter()
                .into(itemView.iv_picture_cart)
            itemView.tv_name_cart.text = productEntity.name
            itemView.tv_description_cart.text = productEntity.description
            itemView.tv_price_cart.text = "$priceRupiah руб"
            itemView.tv_value_cart.text = productEntity.qty.toString()
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CartViewHolder {
        val view: View =
            LayoutInflater.from(parent.context).inflate(R.layout.item_cart, parent, false)
        return CartViewHolder(view)
    }

    override fun onBindViewHolder(holder: CartViewHolder, position: Int) {
        holder.bind(list[position])
    }

    override fun getItemCount(): Int {
        return list.size
    }

    fun setDataAdapter(data: List<ProductEntity>) {
        list.clear()
        list.addAll(data)
        notifyDataSetChanged()
        val total = list.sumBy{it.priceToQty}
        listener.onTotalChange(total)
    }

}

ProductEntity.kt - product model.

@Parcelize
@Entity(tableName = "product")
data class ProductEntity(
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name="pk")
    val pk: Int = 0,
    @ColumnInfo(name="id")
    val id: Int = 0,
    @ColumnInfo(name= "name")
    val name: String = "",
    @ColumnInfo(name= "picture")
    val picture: Int = 0,
    @ColumnInfo(name= "description")
    val description: String = "",
    @ColumnInfo(name= "price")
    val price: Int = 0,
    @ColumnInfo(name= "qty")
    val qty: Int = 0,
    @ColumnInfo(name = "type")
    val type: Int = ProductSavedType.FAV

): Parcelable{

    val priceToQty get() = qty * price
}