Kotlin Multiplatform Main Activity layout

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">

    <Button android:id="@+id/red_button"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:text="@string/red_button"
            android:layout_margin="16dp"
            android:textColor="#FF0000"
            android:onClick="onButtonClick"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"/>

    <Button android:id="@+id/green_button"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:text="@string/green_button"
            android:layout_margin="16dp"
            android:textColor="#00FF00"
            android:onClick="onButtonClick"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/red_button"/>

    <Button android:id="@+id/blue_button"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:text="@string/blue_button"
            android:layout_margin="16dp"
            android:textColor="#0000FF"
            android:onClick="onButtonClick"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/green_button"/>

</android.support.constraint.ConstraintLayout>