QtQuick.Controls.Impl not found

[jaquilio]

When I load back a project, an error message saying the following briefly appears.

Line 10: Implicit import of QtQuick.Controls.impl of Qml module 'QtQuick.Controls' not found.

What causes this error and how can it be fixed?
It is not an import I added myself.
The error message appears only for one or two seconds and then disappears. The error is indeed genuine in that the loaded UI has some elements missing.

Qt Design Studio Version I am using: 3.6.0.