1. Open a QML file in the Designer and select an Image item.
2. In the Properties view, try to drop down the "Source" combo box.
   All you'll see is a gray line:
   
3. Select a different window, e.g. Firefox, using Alt+Tab.
4. Try to come back to Creator.
   Creator won't react on any input anymore.

The dropdown should display some useful data. If the dropdown is not meant to display anything, use an edit instead of a combobox.
Even more important: Creator should always remain responsive.