panic: Session already closed in presence watcher

[Dave Cheney]

panic: Session already closed

goroutine 1721 [running]:
runtime.panic(0x7c1b80, 0xc21036a950)
 /usr/lib/go/src/pkg/runtime/panic.c:266 +0xb6
gopkg.in/mgo%2ev2.(*Session).cluster(0xc210621380, 0x2b5beeed9050)
 /home/ubuntu/juju-core_1.26-alpha1/src/gopkg.in/mgo.v2/session.go:1438 +0x66
gopkg.in/mgo%2ev2.copySession(0xc210621380, 0x1, 0xc210387540)
 /home/ubuntu/juju-core_1.26-alpha1/src/gopkg.in/mgo.v2/session.go:500 +0x32
gopkg.in/mgo%2ev2.(*Session).Copy(0xc210621380, 0x24)
 /home/ubuntu/juju-core_1.26-alpha1/src/gopkg.in/mgo.v2/session.go:1404 +0x41
github.com/juju/juju/state/presence.(*Watcher).sync(0xc21061edc0, 0x0, 0x0)
 /home/ubuntu/juju-core_1.26-alpha1/src/github.com/juju/juju/state/presence/presence.go:386 +0x1b9
github.com/juju/juju/state/presence.(*Watcher).loop(0xc21061edc0, 0xc210622d20, 0xc21062a000)
 /home/ubuntu/juju-core_1.26-alpha1/src/github.com/juju/juju/state/presence/presence.go:263 +0x1cb
github.com/juju/juju/state/presence.func·001()
 /home/ubuntu/juju-core_1.26-alpha1/src/github.com/juju/juju/state/presence/presence.go:139 +0x36
created by github.com/juju/juju/state/presence.NewWatcher
 /home/ubuntu/juju-core_1.26-alpha1/src/github.com/juju/juju/state/presence/presence.go:149 +0x1a8

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