qml file cannot be opened in the design

import QtQuick
import QtQuick.Controls
import "../controls"

Item {
    // Properties
    property string getText: textArea.text
    property string setText: ""

    Rectangle {
        id: rectangle
        color: "#2c313c"
        anchors.fill: parent

        Flickable {
            id: flickable
            anchors.fill: parent
            clip: true

            TextArea.flickable: TextArea{
                id: textArea
                background: Rectangle {
                    anchors.fill: parent
                    color: "#2c313c"
                }
                padding: 10
                wrapMode: Text.WordWrap
                textFormat: Text.AutoText
                placeholderTextColor: "#f6fbff"
                selectByMouse: true
                selectedTextColor: "#ffffff"
                selectionColor: "#ff007f"
                color: "#ffffff"
                font.pointSize: 12
                text: setText
            }

            ScrollBar.vertical: CustomScrollBar {} // No error is reported if using the scrollbar  

        }
    }

}

import QtQuick
import QtQuick.Controls

// ScrollBar consists of two visual items: background and content item.
ScrollBar {
                id: scrollBar
                active: true
                minimumSize: 0.3
                position: 0.2

                background: Rectangle {
                    anchors.fill: parent
                    color: "#2c314c"
                }

                contentItem: Rectangle {
                        implicitWidth: 10
                        implicitHeight: 100
                        radius: 6
                        color: scrollBar.pressed ? "#81e889" : "#c2f4c6"
                        // Hide the ScrollBar when it's not needed.
                        opacity: scrollBar.policy === ScrollBar.AlwaysOn || (scrollBar.active && scrollBar.size < 1.0) ? 0.75 : 0

                        // Animate the changes in opacity (default duration is 250 ms).
                        Behavior on opacity {
                            NumberAnimation {}
                        }
                    }
}