It's not possible to save a Form in Form Builder, trying to close Form Builder discards the draft.
Steps to reproduce:
1. Login as admin
2. Go to Content -> Forms
3. Start creating a new Form
4. Open Form Builder (click the "Building form" button)
5. Add a button and configure it
6. Click the "X" in the top left cornet
I'm back to Content Edit view, form preview is displayed
Clicking "X" discards the draft
See screenshot to see how it looks and how it should look - looks like there is a CSS issue there.