PreferenceFieldDelegate.kt

package br.com.ilhasoft.clickcondo.helpers

import br.com.ilhasoft.support.core.preferences.BasePreferences
import kotlin.reflect.KProperty

/**
 * Created by john-mac on 11/23/16.
 */
class PreferenceFieldDelegate<T>(val defaultValue: T?) {

    @Suppress("UNCHECKED_CAST")
    operator fun getValue(basePreferences: BasePreferences, propertyMetadata: KProperty<*>): T? {
        return basePreferences.getValue(propertyMetadata.name, defaultValue)
    }

    operator fun setValue(basePreferences: BasePreferences, propertyMetadata: KProperty<*>, a: Any?) {
        if (a != null) {
            basePreferences.setValue(propertyMetadata.name, a)
        }
    }
}

Talvez fosse mais interessante usar assim a?.let { basePreferences.setValue(propertyMetadata.name, it) } , vai dar no mesmo no fim das contas.