Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

Null Safety

Because Kotlin distinguishes between nullable and non-nullable types, the compiler is aware of the possibly dangerous situation of asking a variable defined as a nullable type to do something when the variable might not exist. To shield against these dangers, Kotlin will prevent you from calling functions on a value defined as nullable until you have accepted responsibility for this unsafe situation.

To see what this looks like in practice, reinstate your call to replace on quest:

Listing 7.5  Using a nullable variable (Main.kt)

...
private fun readBountyBoard() {
    val quest: String? = obtainQuest(playerLevel)
    val censoredQuest = quest.replace("Nogartse", "xxxxxxxx") println( """ |$HERO_NAME approaches the bounty board. ...