Toggling visibility in Kotlin, isVisible unresolved reference

Slowly learning Kotlin. Just generating a random number from a roll. If roll = 9 I want to make the button and seekbar invisible.

I'm using the toggleVisibility function to accomplish this, but the Kotlin compiler sees isVisible as a unresolved reference

package com.example.randomizer

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.SeekBar
import android.widget.TextView
import android.widget.VideoView
import java.util.*

class MainActivity : AppCompatActivity() {

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

        val rollButton = findViewById<Button>(R.id.rollButton)
        val resultsTextView = findViewById<TextView>(R.id.resultsTextView)
        val seekBar = findViewById<SeekBar>(R.id.seekBar)
        val winText = "9 You Win !"




        rollButton.setOnClickListener {
            val rand = Random().nextInt(seekBar.progress)
            resultsTextView.text = rand.toString()
            if (rand == 9) {
                resultsTextView.text = winText
                seekBar.toggleVisibility()
                rollButton.toggleVisibility()
            }

        }



    }

    fun View.toggleVisibility() {
        if (this.isVisible()) {
            this.visibility = View.INVISIBLE
        } else {
            this.visibility = View.VISIBLE
        }
    }
}

Compiler error:

unresolved reference isVisible

Did you define isVisible for a View by yourself?
View class has no method called isVisible().

As @user2340612 said, it can be defined as:

fun View.isVisible(): Boolean {
    return this.visibility == View.VISIBLE
}