How to fix this bug in the Kotlin Web Demo?

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I'm a real n00b in Kotlin and have just started with its demo. There' no direct link, please click on Problems and then on Runs on the left.

The problem solution is trivial, but there's a bug in

private fun assertEquals<T>(actual : T?, expected : T?, message : Any? = null) {
    if (actual != expected) {
        errors++
        println("Test failed")
        val trace = Thread.currentThread()?.getStackTrace()!!
        if (trace.size > 6) {
            // Finding relevant stack frames
            val location = trace.getFrameAfter("runs.Tester", "expect") // ERROR HERE
            val function = trace.getFrameAfter("runs.TesterRunner", "forFunction") // AND HERE
            println("at ${function?.getClassName()}.${function?.getMethodName()}(line:${location?.getLineNumber()})")
        }
        if (message != null)
            println(message)
    }
    else if (!skipSuccessful)
        println("OK")
}

which I don't understand. It says

 Type mismatch: inferred type is kotlin.Array<java.lang.StackTraceElement> but
 kotlin.Array<java.lang.StackTraceElement?> was expected

and I can neither how the former was inferred nor why the latter is expected. Especially I don't understand where can two such assumption come from a single method call.

I "fixed" it by removing the offending lines, but I'm sure someone can enlighten me.

asked 43 secs ago

maaartinus

13k

How to fix this bug in the Kotlin Web Demo?