QCreator in design mode doesn't show my window

faezehFrj

I am very new in QCreator, I create new project an I want to edit my project in design mode but unfortunately when I clicked on this item, sudden 3 windows open same time. when I close one of them, all windows close and I don't have white window for attach my item such as rectangles and text filed, finally I don't know what am i doing now.

my code is:

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    visible: true
    color: "#55aa7f"
    width: 100
    height: 200
    title: qsTr("Hello World")
}