Regression: Black scene after QQuickWidget closed and shown

When closing an widget containing a QQuickWidget and opening it again the scene becomes black.

This is a major regression since this does not happen with Qt 5.12.3.

See attached example application.

The following steps are used to reproduce:

1. Build and run the example with Qt 5.12.4
2. Push the button to open the Widget
3. Close the Widget that opened by pressing the "x" button
4. Press the button again to re-open the widget. 

Expected behaviour: The widget should re-open with the list shown on the widget.

Actual behaviour: The widget is displayed with a black scene. See attached screenshot.

Repeat the above steps but this time using Qt 5.12.3. The widget is shown with the correct information after re-opening.