It’s hard to imagine a performer like Stevie Nicks ever gets nervous, even playing to roughly 20,000 faces. But the “Gypsy” singer openly admitted as much a couple times during her Wednesday performance at Brooklyn’s Barclays Center, once after she started an anecdote about “Stop Draggin’ My Heart Around,” the Tom Petty duet she wasn’t actually meant to queue up until a little farther into the setlist. Nerves are somewhat understandable though.