QT5 - Promoting widgets makes the form designer unusable?

[Jufa]

Hello there!

I have QT5 with QTCreator. At this point, I can't create my own widgets as plugins, so I'm starting to use the "promote widget" way. But, this way, I can't use the form designer to create my project. I can't see how my forms looks until I run my application. I only see a collection of empty qwidgets.

1-Is this the only way to go?
2-Is there any way to preview the promoted widgets in design time?
3-Is there a way to create your own widgets as plugins for qtCreator in QT5?

Thanks in advance for your responses and your time!