Android Kotlin findViewById must not be null

As @Declan Nnadozie already mentioned: btnYES = findViewById(R.id.btnYES) returns null because btnYES is not a view inside the contentView inflated to PageTwoActivity.
The buttons btnYES and btnNO and the EditText etStudentName can be found in the content that is inflated in the dialog:
Also in Kotlin you do not need findViewById to access the activity's views.
You can delete all these:

internal lateinit var btnBACK: Button
internal lateinit var btnDIALOG: Button

internal lateinit var btnYES: Button
internal lateinit var btnNO: Button
internal lateinit var etStudentName:EditText

My suggestion is to use the below code:

class PageTwoActivity : AppCompatActivity() {
var EnteredText: String = ""

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

        addListenerOnButtonBACK()
        addListenerOnButtonDIALOG()

        Toast.makeText(this@PageTwoActivity, "You are on Page Two",
            Toast.LENGTH_LONG).show()
    }

    fun doCustom(v: View) {
        val openDialog = Dialog(this)
        openDialog.setContentView(R.layout.custom_dialog)
        val btnYES = openDialog.findViewById<Button>(R.id.btnYES)
        val btnNO = openDialog.findViewById<Button>(R.id.btnNO)
        val etStudentName = openDialog.findViewById<EditText>(R.id.etStudentName)
        openDialog.setCancelable(false)

        btnYES.setOnClickListener(View.OnClickListener {
            EnteredText = etStudentName.getText().toString().trim({ it <= ' ' })
            if (EnteredText.isEmpty()) {
                Toast.makeText(applicationContext, "Enter Your Name\n\n OR Click DECLINE", Toast.LENGTH_LONG).show()
                        return@OnClickListener
            }
            Toast.makeText(applicationContext, "Registered $EnteredText", Toast.LENGTH_SHORT).show()
            openDialog.dismiss()
        })

        btnNO.setOnClickListener(View.OnClickListener {
            EnteredText = ""
            val intent = Intent(this@PageTwoActivity, MainActivity::class.java)
            startActivity(intent)
            openDialog.dismiss()
        })
        openDialog.show()
    }