moja wersja:
        val negative =
            View.OnClickListener { v ->
                val b =
                    v as Button
                if (newNumber.toString() == ""){
                    newNumber.append("-")

wersja z neta:
      buttonNeg.setOnClickListener { view ->
            val value = newNumber.text.toString()
            if(value.isEmpty()){
                newNumber.setText("-")
            }else{
                var doubleValue = value.toDouble()
                doubleValue *= -1
                newNumber.setText(doubleValue.toString())
            }
        }