Qt5.8: macOS, designer examples fails to compile

qt-enterprise-mac-x64-clang-5.8.0-rc_657.dmg

Install with default options, launch QtCreator and open one of the designer examples, for example designer/containerextension. Build fails with error message:
fatal error: 'QtUiPlugin/QDesignerExportWidget' file not found

Same happends with following examples (all try to include #include <QtUiPlugin/QDesignerCustomWidgetInterface>):
customwidgetplugin
taskmenuextension
worldtimeclockplugin

(calculatorbuilder, calclatorform and worldtimeclockbuilder examples work fine)

containerextension example tested also on Ubuntu14.04, works fine there.