1. Easier to add new customer and items + cleaner look
When you create an invoice or any other document, and you want to quickly add a new customer or item to your database, you will see a new button for that.
This feature already exists, this is just a new design, which gives cleaner look to the creator pages.
2. Warning message
When you create an invoice or any other document, and you don't select a customer or an item, and you press any f the save buttons, you will see a kind message which alerts you to select at least a customer and an item.