QWidgetAction creates empty dummy window on macOS 10.12.6

I would like to use a custom widget in the context menu of a menu bar icon. If I add this custom widget (e.g. a simple QPushButton) to the context menu via QWidgetAction and open it, an empty dummy window appears (see the attached screenshot). If i close that window, the custom widget disappears from the context menu.

Qt 5.7 and 5.8 seem to be unaffected.

macOS 10.13 affected as well.

Steps to reproduce on macOS 10.12.6:

• Download and compile the attached test application
• Run the application
• Click on the small "Qt" logo on the menu bar
• The context menu appears with a button in it

Expected: no additional window is visible

Actual: there is a small empty window in the center of the screen