Saving a pattern swatch

Monika is right, but I want to add to this thread. Another way to save your linework with the live brush applied (before it gets expanded) is to copy it to your clipboard and paste on the artboard when you leave Pattern mode. Making a symbol out of the art saves it to your symbols panel and you can get it there and edit it or break the link and use it as an object. But the symbol content that is inside your pattern mode pattern will be expanded on exit, just like any other "live" content. 

I think the symbol workaround you described is a great idea for saving your original live art apart from your pattern fill when you are drawing in Pattern Mode and you get that warning. I just want to clarify that it does not protect the art from getting expanded 'within' the pattern fill. But it does create a copy of the live art that you can change the brushstroke on later. So for pattern designers, I always recommend keeping copies of your art/motifs outside of pattern mode so you retain all of it's editability should you need to change a brushstroke or thickness later.