How to use reduce function correctly with functional programming in kotlin

I am trying to pass a function and want to use reduce on it. This is the data class

data class TestClass(
   var variable1: BigDecimal?,
   var variable2: BigDecimal?
)

This is the function which I want to work

fun aggregateAll() {
   helperfunc(TestClass::variable1::get,someDummmyFilledListOfTestClass)
}

fun helperfunc(function: () ->BigDecimal, testList:List<TestClass>) {
     var aggregatedValue:BigDecimal = BigDecimal.ZERO
     testList.map{function}.reduce{aggregatedValue,element -> aggregatedValue.add(element)}
}

fun main() {
    val someDummmyFilledListOfTestClass = listOf<TestClass>(
        TestClass(1.toBigDecimal()),
        TestClass(199.toBigDecimal()),
        TestClass(null),
        TestClass(501.toBigDecimal())
    )
    val result = helperfunc(someDummmyFilledListOfTestClass, TestClass::variable1::get)
                                                                 // trailing lambda
    // or val result = helperfunc(someDummmyFilledListOfTestClass) { it.variable1 }
    println(result)
}
                                         /* lambda last parameter for trailing lambda */
fun helperfunc(testList: List<TestClass>, function: (TestClass) -> BigDecimal?): BigDecimal {
    return testList
        // map TestClass to BigDecimal? and filter nulls
        .mapNotNull(function)
        // fold with initial value, reduce will throw exception if list is empty
        .fold(BigDecimal.ZERO) { acc, element -> acc.add(element) }
}

data class TestClass(
    var variable1: BigDecimal?
//  var variable2: BigDecimal?
)