New dockwidgets not editable in designer

When a new QDockWidget is added to a project by

"Add new..." --> "Qt" --> "Qt Designer Form Class" --> "Widgets" --> "QDockWidget" 

the resulting class is not editable in the graphical designer.

The cause of this seems to be a missing line in the ui file, typically

<widget class="QWidget" name="dockWidgetContents"/>

See also: https://stackoverflow.com/questions/47914013/qdockwidget-form-in-qt-creator-designer