Default group choice causes null pointer exception

[FWDekker]

If a command has a group choice with a default group, then the options in this group do not work correctly if the user does not specify the group choice explicitly.

Expectation

Consider the following minimal working example.

class Command : CliktCommand() {
    class GroupA : OptionGroup() {
        val subOptionA: Int by option()
            .int()
            .default(5)
    }
    class GroupB : OptionGroup()

    val groupOption by option()
        .groupChoice("a" to GroupA(), "b" to GroupB())
        .defaultByName("a")

    override fun run() {
        when (val it = groupOption) {
            is GroupA -> println(it.subOptionA)
            is GroupB -> println("Nothing to see here")
            else -> error("Oops")
        }
    }
}

fun main(args: Array<String>) = Command().main(args)

The default choice for groupOption is a. Therefore, I expect that if --group-option is not specified, the behaviour is equivalent to specifying --group-option a. This means that subOptionA takes on the value 5, and running the program should print 5.

Reality

The above code throws an exception if no arguments are provided to the program. Similarly, running with only the arguments --sub-option-a 3 also throws an exception. Only when --group-option a is also specified does the program work without exceptions.

Not giving any exceptions results in the following trace.

Exception in thread "main" java.lang.NullPointerException
    at com.fwdekker.mypackage.Command$GroupA.getSubOptionA(Main.kt)
    at com.fwdekker.mypackage.Command.run(Main.kt:47)
    at com.github.ajalt.clikt.parsers.Parser.parse(Parser.kt:168)
    at com.github.ajalt.clikt.parsers.Parser.parse(Parser.kt:16)
    at com.github.ajalt.clikt.core.CliktCommand.parse(CliktCommand.kt:258)
    at com.github.ajalt.clikt.core.CliktCommand.parse$default(CliktCommand.kt:255)
    at com.github.ajalt.clikt.core.CliktCommand.main(CliktCommand.kt:273)
    at com.github.ajalt.clikt.core.CliktCommand.main(CliktCommand.kt:298)
    at com.fwdekker.mypackage.MainKt.main(Main.kt:54)

Additional information

System	Version
Operating system	Windows 10 or Ubuntu 18.04
Kotlin	1.3.72
Java	1.8.0 or 11.0.6

[FWDekker]

Awesome! Thanks!

[ajalt]

Thank you for the clear write-up! I just released version 2.7.1, which contains this fix.