singleOrNull

Returns single character, or null if the char sequence is empty or has more than one character.

fun CharSequence.singleOrNull(): Char?(source)

fun main() {
    val singleChar: CharSequence = "A"
    val empty: CharSequence = ""
    val multipleChars: CharSequence = "AB"

    println(singleChar.singleOrNull())     // prints: A
    println(empty.singleOrNull())          // prints: null
    println(multipleChars.singleOrNull())  // prints: null
}

Source