1 First, i designed three qml components myself, their names are:
MyButton,
MyButton2,
MyExample
ps: See the details in myQml.tar.gz
2 Then, i used them in a Qt Quick Application named untitled5-4-1, but when i want to preview the main.qml in qmldesigner widget, MyButton can not be showed successfully, at the same time MyButton2 can be showed successfully. By the way, this application can run successfully
ps: You can see the details in untitled5-4-1.tar.gz
3 preview.png is the phenomenon