NullPointerException when overridding onSelectionChanged in custom edittext

Question

Any variables that are referenced in onSelectionChanged are null according to the debugger, and if I run my sample app with the following added to a simple layout my app crashes with a nullpointer.

 <com.dummy.DummyEditText
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"/>

The simplest example of a custom AppCompatEditText

package com.dummy

import android.content.Context
import android.util.AttributeSet
import androidx.appcompat.widget.AppCompatEditText

class DummyEditText : AppCompatEditText {

    constructor(context: Context) : super(context)
    constructor(context: Context, attributeSet: AttributeSet) : super(context, attributeSet)
    constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int) : super(context, attrs, defStyleAttr)

    private val a = String()
    override fun onSelectionChanged(selStart: Int, selEnd: Int) {
        a.toString()
    }
}

with the stacktrace

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toString()' on a null object reference
        at com.dummy.DummyEditText.onSelectionChanged(DummyEditText.kt:15)
2020-10-05 12:14:25.794 22859-22859/com.dummy E/AndroidRuntime:     at android.widget.TextView.spanChange(TextView.java:10744)
        at android.widget.TextView$ChangeWatcher.onSpanAdded(TextView.java:13600)
        at android.text.SpannableStringBuilder.sendSpanAdded(SpannableStringBuilder.java:1287)
        at android.text.SpannableStringBuilder.setSpan(SpannableStringBuilder.java:777)
        at android.text.SpannableStringBuilder.setSpan(SpannableStringBuilder.java:676)
        at android.text.Selection.setSelection(Selection.java:96)
        at android.text.Selection.setSelection(Selection.java:78)
        at android.text.Selection.setSelection(Selection.java:153)
        at android.text.method.ArrowKeyMovementMethod.initialize(ArrowKeyMovementMethod.java:312)
        at android.widget.TextView.setText(TextView.java:6299)
        at android.widget.TextView.setText(TextView.java:6139)
        at android.widget.EditText.setText(EditText.java:121)
        at android.widget.TextView.<init>(TextView.java:1642)
        at android.widget.EditText.<init>(EditText.java:87)
        at android.widget.EditText.<init>(EditText.java:83)
        at androidx.appcompat.widget.AppCompatEditText.<init>(AppCompatEditText.java:74)
        at androidx.appcompat.widget.AppCompatEditText.<init>(AppCompatEditText.java:69)
        at com.dummy.DummyEditText.<init>(DummyEditText.kt:10)

So the question is why is the variable a null? It should have been instantiated when it is first referenced in the onSelectionChanged regardless of how DummyEditText is instantiated. What am I missing here? Any pointers on what is going on are appreciated!