assistant: Fix button sensitive during box creation

When the user presses the "Create" button, it needs a couple of seconds to prepare and evaluate everything, and only then the dialog is closed. During this time, the "Create" button is sensitive, which means that the user can press it again, triggering the creation process again.

This patch disables the button during this process. This has two advantages:

• Avoids pressing the button twice
• Gives visual confirmation to the user that they has pressed the button correctly, and the process has begun.