QFormBuilder was unable to create a widget of the class 'WorldTimeClock'."

[Panoss]

I opened worldtimeclockbuilder and worldtimeclockplugin from Qt Creator 's exxamples and I run worldtimeclockbuilder.
It run but returned me the folloeing error:
"QFormBuilder was unable to create a widget of the class 'WorldTimeClock'."
"Empty widget item in QHBoxLayout ''."

The form shoed up but without the worldtimeclockplugin in it.
How can I fix it? (Qt Creator 2.7.0)