fix: add lock to terminate handler

close_handler.go

@@ -1,6 +1,9 @@
 package grawt
 
+import "sync"
+
 type CloseHandler struct {
+	sync.Mutex
 	waiter      *Waiter
 	Quit        chan struct{}
 	active      bool

waiter.go

@@ -16,11 +16,11 @@ type Waiter struct {
 
 func (w *Waiter) addHandler(f *func(), autoDone bool) *CloseHandler {
 	ch := CloseHandler{
-		w,
+		waiter:      w,
-		make(chan struct{}, 1),
+		Quit:        make(chan struct{}, 1),
-		true,
+		active:      true,
-		autoDone,
+		autoDone:    autoDone,
-		f,
+		handlerFunc: f,
 	}
 	w.waitGroup.Add(1)
 	w.closeHandlers = append(w.closeHandlers, &ch)
@@ -32,11 +32,15 @@ func (w *Waiter) terminateHandler(h *CloseHandler, forceWaitGroupDone bool) {
 	if h.handlerFunc != nil && *h.handlerFunc != nil {
 		(*h.handlerFunc)()
 	}
-	close(h.Quit)
+	h.Lock()
+	if h.active {
+		close(h.Quit)
+	}
 	if h.autoDone || forceWaitGroupDone {
 		w.waitGroup.Done()
 	}
 	h.active = false
+	h.Unlock()
 }
 
 func (w *Waiter) AddCloseHandler(f func(), waitForChannel bool) *CloseHandler {