Form Editor not working

I am not able to use the Form Editor of the Design tool for QML projects.

I'm using a fresh installation of Qt Creator installed with Qt 5.14.0. The problem applyes also for empty projects. There are the steps I'm following. From File -> New File or Project... I generate a new QtQuick Application - Empty. I leave all the other settings to the default values. The used kit is Desktop Qt 5.14.0 MSVC2015 64-bit.

The content of the automatically generated main.qml is

import QtQuick 2.12
import QtQuick.Window 2.12

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

When I try to edit ir in the Design tool, the content of the Form Editor replaced by this box:

The link points to the first line of main.qml.

Even if I'm not able to use the Form Editor, the project compiles correctly and an empty window named Hello World is shown.