Kotlin - I get error beacuse id is null. How to fix it?

I'm trying to make a program which will display color that you choose. So you would slide on picture with color shades and application would display you current color. Under that there is text view that will display RGB value of current color. But when I tried to write code for color picker I get this error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.leds/com.example.leds.MainActivity}:
java.lang.NullPointerException: color_picker must not be null

I tried everything but i can't fix it.

My kotlin code:

class MainActivity : AppCompatActivity() {

private lateinit var bitmap:Bitmap

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

    color_picker.isDrawingCacheEnabled = true
    color_picker.buildDrawingCache(true)

    val wheel = findViewById<ImageView>(R.id.color_picker)

    wheel.setOnTouchListener { v, event ->
        if (event.action == MotionEvent.ACTION_DOWN || event.action == MotionEvent.ACTION_MOVE) {
            bitmap = color_picker.drawingCache

            val pixel = bitmap.getPixel(event.x.toInt(), event.y.toInt())

            val r = Color.red(pixel)
            val g = Color.green(pixel)
            val b = Color.blue(pixel)

            selected_color_view.setBackgroundColor(Color.rgb(r,g,b))

            color_value.text = "RGB: $r, $g, $b"
        }
        true}

And my xml code:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:gravity="center">

<ImageView
    android:id="@+id/color_picker"
    android:layout_width="350dp"
    android:layout_height="450dp"
    android:src="@drawable/colour_wheel"/>

<View
    android:id="@+id/selected_color_view"
    android:layout_width="100dp"
    android:layout_height="100dp" />

<TextView
    android:id="@+id/color_value"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="RGB:"
    android:textColor="#000"
    android:textSize="18sp"/>