[android] QQuickWidget in a QDialog shows the contents of a qml file that was shown earlier by a different QQuickWidget

Scenario:

• app using two dialog widget classes (inheriting QDialog).
• both dialogs use a QQuickWidget.
• each QQuickWidget use a different qml source file.
• first one of the dialogs is created and shown, and after that closed.
• then the second dialog is created and shown.
• unexpectedly the QQuickWidget of the second dialog seems to show the contents of a wrong qml file, namely the contents of the qml file used by the first dialog.

Steps to reproduce:

• run the sample on an Android device (not all devices reproduce the issue: Nexus 7 with android 4 or 5 reproduces, Pixel C with android 8.1.0 doesn't reproduce).
• press either one of the buttons -> memorize the color of the rectangle shown on the dialog.
• press the top of the dialog to return to main window.
• press the other button -> observe the color.
  EXPECTED: The color should be different. (The other qml file has a red rectangle and the other one has a blue rectangle).
  ACTUAL: The color doesn't change -> the dialog opened second seems to show the contents of the wrong qml file.