How to fix Unresolved reference: mInterstitialAd 'declare the same variable in another class in Kotlin'

I want to display interstitial ads but it won't because it's in another class (adapter class). How do I call the same variable in another class?

This is how my code looks like:

class MainActivity : AppCompatActivity() {

    private lateinit var mInterstitialAd: InterstitialAd
    var adapter:ChaptersAdapter?=null
    var listOfChapters= ArrayList<Chapters>()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        MobileAds.initialize(this,
            "ca-app-pub-3940256099942544~3347511713")

    MobileAds.initialize(this,
        "ca-app-pub-3940256099942544~3347511713")

        mInterstitialAd = InterstitialAd(this)
        mInterstitialAd.adUnitId = "ca-app-pub-3940256099942544/1033173712"
        mInterstitialAd.loadAd(AdRequest.Builder().build())

        mInterstitialAd.adListener = object : AdListener() {
            override fun onAdClosed() {
                val context = null
                val intent = Intent (context, ChapterDetails::class.java)
                        startActivity(intent)
                mInterstitialAd.loadAd(AdRequest.Builder().build())
            }
        }

        loadChapters()
        adapter=ChaptersAdapter(listOfChapters,this)
        lvchapters.adapter =adapter
    }

    fun loadChapters(){
        listOfChapters.add(Chapters("", applicationContext.assets.open    ("Chapter0.txt").bufferedReader().use {
            it.readText()
        }
        ))

    class ChaptersAdapter: BaseAdapter {
        var context:Context?=null
        var listOfChaptersLocal= ArrayList<Chapters>()
        constructor(listOfChapters:ArrayList<Chapters>,context:Context){
            listOfChaptersLocal=listOfChapters
            this.context=context
        }

        override fun getView(p0: Int, p1: View?, p2: ViewGroup?): View {
            val chapters= listOfChaptersLocal[p0]
            var inflator= context!!.getSystemService    (Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
            val chaptersView=inflator.inflate(R.layout.list_chapters,null)
            chaptersView.tvName.text= chapters.name!!
            chaptersView.tvName.setOnClickListener {
                val intent =Intent(context,ChapterDetails::class.java)
                if (***mInterstitialAd***.isLoaded) {
                    ***mInterstitialAd***.show()
                } else {
                    intent.putExtra("name",chapters.name!!)
                    intent.putExtra("des",chapters.des!!)
                    context!!.startActivity(intent)
                }
            }
            return chaptersView
        }

        override fun getItem(p0: Int): Any {
            return listOfChaptersLocal[p0]
        }

        override fun getItemId(p0: Int): Long {
            return p0.toLong()
        }

        override fun getCount(): Int {
            return listOfChaptersLocal.size
        }
    }
}

When I build the project I get the message:

'Unresolved reference: mInterstitialAd'

I have tried deferent methods but without any result it keep telling me that, what should I do?