I have this QML files

//main.qml

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.2

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Component {
        id: firstPage

        BasePage {
            bottomBar.rightButton.text: qsTr("Next")
            bottomBar.rightButton.visible: true
            bottomBar.visible: true

            Button {
                height: parent.height * 0.2
                width: parent.width * 0.3
                anchors.centerIn: parent
                text: "NEXT"
                onClicked: stackView.push(secondPage)
            }
        }
    }

    Component {
        id: secondPage

        BasePage {
            onBackKeyPressed: stackView.pop()
        }
    }

    StackView {
        id: stackView
        initialItem: firstPage
        anchors.fill: parent
    }
}

//BasePage.qml

import QtQuick 2.9
import QtQuick.Controls 2.2
import QtQuick.Layouts 1.3

Page {
    default property alias content: contentItem.data
    property alias bottomBar: bottomBar
    signal bottomBarLeftButtonClicked()
    signal bottomBarCentralButtonClicked()
    signal bottomBarRightButtonClicked()
    signal backKeyPressed()

    id: basePageItem
    focusPolicy: Qt.ClickFocus

    Keys.onReleased: {
        if(event.key !== Qt.Key_Back && event.key !== Qt.Key_Escape)
            return;

        backKeyPressed()
        event.accepted = true
    }

    Rectangle {
        id: background
        anchors.fill: parent

        ColumnLayout {
            id: basePage
            anchors.fill: parent
            spacing: 0

            Item {
                id: contentItem
                Layout.fillHeight: true
                Layout.fillWidth: true
            }

            BottomBar {
                id: bottomBar
                Layout.preferredHeight: parent.height * 0.1
                Layout.preferredWidth: parent.width
                visible: leftButton.visible || centralButton.visible || rightButton.visible
                onLeftButtonClicked: bottomBarLeftButtonClicked()
                onCentralButtonClicked: bottomBarCentralButtonClicked()
                onRightButtonClicked: bottomBarRightButtonClicked()
            }
        }
    }
}

//BottomBar.qml

import QtQuick 2.9
import QtGraphicalEffects 1.0
import QtQuick.Controls 2.2
import QtQuick.Layouts 1.3

Rectangle {
    property alias leftButton: leftButton
    property alias centralButton: centralButton
    property alias rightButton: rightButton

    signal leftButtonClicked()
    signal centralButtonClicked()
    signal rightButtonClicked()

    id: bottomBar
    color: "#fafafa"

    ColumnLayout {
        anchors.fill: parent
        spacing: 0

        LinearGradient {
            Layout.preferredHeight: parent.height * 0.1
            Layout.preferredWidth: parent.width
            gradient: Gradient {
                GradientStop { position: 0.0; color: "#fafafa" }
                GradientStop { position: 1.0; color: "#d7d7d7" }
            }
        }

        Item {
            Layout.preferredHeight: parent.height * 0.9
            Layout.preferredWidth: parent.width

            RowLayout {
                anchors.fill: parent
                spacing: width * 0.02

                Item {
                    Layout.preferredHeight: parent.height
                    Layout.preferredWidth: parent.width * 0.32
                    BottomBarItem {
                        id: leftButton
                        visible: false
                        anchors.fill: parent
                        onClicked: bottomBar.leftButtonClicked()
                    }
                }


                Item {
                    Layout.preferredHeight: parent.height
                    Layout.preferredWidth: parent.width * 0.32
                    BottomBarItem {
                        id: centralButton
                        visible: false
                        anchors.fill: parent
                        onClicked: bottomBar.centralButtonClicked()
                    }
                }

                Item {
                    Layout.preferredHeight: parent.height
                    Layout.preferredWidth: parent.width * 0.32
                    BottomBarItem {
                        id: rightButton
                        anchors.fill: parent
                        visible: false
                        onClicked: bottomBar.rightButtonClicked()
                    }
                }
            }
        }
    }
}

//BottomBarItem.qml

import QtQuick 2.9
import QtQuick.Controls 2.2

Item {
    property alias text: buttonText.text
    signal clicked()

    id: item

    Column {
        anchors.fill: parent

        Item {
            height: parent.height * 0.6
            width: height
            anchors.horizontalCenter: parent.horizontalCenter
        }

        Text {
            id: buttonText
            height: parent.height * 0.4
            anchors.horizontalCenter: parent.horizontalCenter
        }
    }

    MouseArea {
        anchors.fill: parent
        onClicked: item.clicked()
    }
}

When the program starts everything works well and the bottomBar is visible.
Then I click on the NEXT button -> second page is shown.
Then I go back to the first page and the bottom bar has disappeared.
Am I doing something wrong?