lib/ui: fix UI.Exit race condition

UI.Exit can be accessed from goroutines drawing, goroutines executing
commands and goroutines waiting for events.

    Write at 0x00c0002b2040 by main goroutine:
      main.main.func1()
          /home/simon/src/aerc2/aerc.go:76 +0x33d
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).BeginExCommand.func1()
          /home/simon/src/aerc2/widgets/aerc.go:245 +0x89
      git.sr.ht/~sircmpwn/aerc2/widgets.(*ExLine).Event()
          /home/simon/src/aerc2/widgets/exline.go:131 +0x442
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).Event()
          /home/simon/src/aerc2/widgets/aerc.go:116 +0x83c
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).simulate()
          /home/simon/src/aerc2/widgets/aerc.go:109 +0x12a
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Aerc).Event()
          /home/simon/src/aerc2/widgets/aerc.go:142 +0x722
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*UI).Tick()
          /home/simon/src/aerc2/lib/ui/ui.go:75 +0x33f
      main.main()
          /home/simon/src/aerc2/aerc.go:94 +0x497

    Previous read at 0x00c0002b2040 by goroutine 19:
      git.sr.ht/~sircmpwn/aerc2/lib/ui.Initialize.func1()
          /home/simon/src/aerc2/lib/ui/ui.go:45 +0x97

    Goroutine 19 (running) created at:
      git.sr.ht/~sircmpwn/aerc2/lib/ui.Initialize()
          /home/simon/src/aerc2/lib/ui/ui.go:44 +0x372
      main.main()
          /home/simon/src/aerc2/aerc.go:87 +0x3a9

1 files changed, 2 insertions, 2 deletions

diff --git a/aerc.go b/aerc.go
index 25a8de64..5b6f9d7f 100644
--- a/aerc.go
+++ b/aerc.go
@@ -73,7 +73,7 @@ func main() {
 					continue
 				}
 			} else if _, ok := err.(commands.ErrorExit); ok {
-				ui.Exit = true
+				ui.Exit()
 				return nil
 			} else if err != nil {
 				return err
@@ -90,7 +90,7 @@ func main() {
 	}
 	defer ui.Close()
 
-	for !ui.Exit {
+	for !ui.ShouldExit() {
 		if !ui.Tick() {
 			// ~60 FPS
 			time.Sleep(16 * time.Millisecond)