Why does my Kotlin program fail to compile?

I'm currently learning Kotlin and using https://play.kotlinlang.org/ to compile my code.
I tried to create a very simple program, collecting user input and giving output.
Howevery, it doesn't run.

fun main() {
   println("Give me a number of minutes!") 
   var input:String = readLine()!!
   var inputInMinutes = input.toInt()
   var resultInSeconds:Int = 0
   resultInSeconds = (inputInMinutes*60)
   println("That is " + resultInSeconds + " seconds!") 
}

I was expecting the program to compile and run, but it displays the following error message:

Give me a number of minutes!
Exception in thread "main" java.lang.NullPointerException
 at FileKt.main (File.kt:7) 
 at FileKt.main (File.kt:-1) 
 at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (:-2)

Is there a mistake in my code or does the online IDE not allow for user input?

Solution

Why does my Kotlin program fail to compile?

Actually it does compile without errors.
An exception like you observed is thrown at runtime, not compile time.

Regarding your specific exception:
According to this post in the site's forum, the Kotlin Playground does not suppport interactive input (like you attempted with readLine()).
Therefore readLine() returns null, and as @KlitosKyriacou rightly pointed out to me readLine()!! which performs a null-assertion then throws the exception you see (originally I mistakenly assumed the exception was thrown by input.toInt()).

A possible solution (as suggested in the post mentioned above) is to write your own function that emulates readLine():

val input = mutableListOf("123") // here you should put your input
fun myReadLine(): String? {
    val line = input.firstOrNull()
    input.removeAt(0)
    return line
}

fun main() {
   println("Give me a number of minutes!") 
   var input:String = myReadLine()!!
   var inputInMinutes = input.toInt()
   var resultInSeconds:Int = 0
   resultInSeconds = (inputInMinutes*60)
   println("That is " + resultInSeconds + " seconds!") 
}

Output:

Give me a number of minutes!
That is 7380 seconds!

Live demo