FlexPanel: Don't crash kwin

FlexPanel widgets can crash kwin.

1. Create a modal popup display in-engine
2. Set the modal to have its window size be controlled by the engine's UI system
3. ADd a root FlexPanel to the window.
4. ADd two child FlexPanel widgets to the root. Set them both to proportional size.
5. Start adding actual widgets to both the child FlexPanel widgets.

Opening the dialog window should immediately crash kwin and the user's entire Plasma session. Wearing my KDE e.V. contractor hat for a second, I have no idea how badly Toolbox is misbehaving to cause that on our end....and this could be kwin's bug. Either way, it's still also a bug in Toolbox.