Crash upon changing opacity after tweaking vertices

[micheus]

Hi kingooze, welcome to our community.

Thanks for report that. I could reproduce it by considering that when you told: "changed the cube into a flat square face" you just scaled or move two faces to overlapped each other (in this case, you turned four faces zero size and that can be the problem - I need to check).

When you did

-cut all the edges by 10
-again, cut all the edges, but now by 2

maybe you already know you could have just used the menu option Cut and chosen the option "Enter number..." entering the value 20.

Ok, you can work on your flower and using the approach you want by using the Extract option - that will create a flat object from the selected faces.
So, you still can add the cube; select one of its face and in the context menu choose Extrac->Normal and [RMB] to accept (without move the mouse). Now you can hide or delete the cube and work in this shape as you tried to do before.
After you get the flower shape you can use Extrude->Normal in order to give it a thickness.