Android Studio: why does the DatePicker function cause my application to crash?

New to Android Studio and Kotlin and I have been working on my new project. I am trying to create a way for users to enter their date of birth using a date picker, but every time I go to that page after setting the date picker, it crashes.

var age_btn: Button? = null
var age_textview: TextView? = null
var cal = Calendar.getInstance()

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


    age_textview!!.text = "--/--/----"

    // create an OnDateSetListener
    val dateSetListener = object : DatePickerDialog.OnDateSetListener {
        override fun onDateSet(view: DatePicker, year: Int, monthOfYear: Int,
                               dayOfMonth: Int) {
            cal.set(Calendar.YEAR, year)
            cal.set(Calendar.MONTH, monthOfYear)
            cal.set(Calendar.DAY_OF_MONTH, dayOfMonth)
            updateDateInView()
        }
    }

    // when you click on the button, show DatePickerDialog that is set with OnDateSetListener
    age_btn!!.setOnClickListener(object : View.OnClickListener {
        override fun onClick(view: View) {
            DatePickerDialog(this@ProfileActivity,
                dateSetListener,
                // set DatePickerDialog to point to today's date when it loads up
                cal.get(Calendar.YEAR),
                cal.get(Calendar.MONTH),
                cal.get(Calendar.DAY_OF_MONTH)).show()
        }

    })

So here is the onDateSet function

private fun updateDateInView() {
    val myFormat = "MM/dd/yyyy" //mention the format you need
    val sdf = SimpleDateFormat(myFormat, Locale.US)
    age_textview!!.text = sdf.format(cal.getTime())
}

I do not understand why the page is blocked, any link to the work date selectors would also be appreciated.