Null vs undefined

craft_model.go

package main

import (
	"gopkg.in/v0/qml"
)

type CraftElement struct {
	Archetype *Archetype
	Ready     bool // True if the player has everything in place to craft this right now
}

type CraftModel struct {
	Len        int
	Craftables []*CraftElement
}

func (cm *CraftModel) AddArchetype(a *Archetype) {
	qml.Lock()

	cm.Craftables = append(cm.Craftables, &CraftElement{a, false})
	cm.Len = len(cm.Craftables)

	qml.Changed(cm, &cm.Len)
	qml.Unlock()
}

func (cm *CraftModel) Get(index int) *CraftElement {
	return cm.Craftables[index]
}

Crafting.qml

import QtQuick 2.1
import QtQuick.Controls 1.1
import QtQuick.Layouts 1.0
// import GoExtensions 1.0


CitWindow {
    id: craftingWindow
    width: 700
    height: 350

    Rectangle {
        id: craftingRect
        clip: true
        x: content_x
        y: content_y
        width: content_width
        height: content_height

        property int selected: 0


        onSelectedChanged: {
            selectionArea.text = "Selected is " + craftingRect.selected
        }

        Item {
            anchors.left: parent.left
            anchors.top: parent.top
            width: parent.width/2
            height: parent.height
            Component {
                id: craftDelegate
                Item {
                    width: parent.width; height: 40
                    Column {
                        Text { text: '<b>Archetype:</b> ' + craftModel.get(index).archetype.simpleName }
                    }
                    MouseArea {
                        id: mouseArea
                        acceptedButtons: Qt.LeftButton | Qt.RightButton
                        width: parent.width
                        height: parent.height
                        hoverEnabled: true         //this line will enable mouseArea.containsMouse

                        onClicked: {
                            craftingRect.selected = index
                            console.log("Selected: " + index + " and crselect: " + craftingRect.selected)
                        }
                    }
                }
            }

            ListView {
                anchors.fill: parent
                model: craftModel.len
                delegate: craftDelegate
                highlight: Rectangle { color: "lightsteelblue"; radius: 5 }
                focus: true
            }
        }

        Item {
            anchors.right: parent.right
            anchors.top: parent.top
            width: parent.width/2
            height: parent.height

            Text {
                id: selectionArea
                text: "............Selected is " + craftingRect.selected
            }

            Button {
                text: "craft!"
                onClicked: {
                    var be = craftModel.get(craftingRect.selected)
                    game.createJob("craft", false, {"product_name": be.archetype.simpleName})
                }
            }

        }
    }
}