Fix crash when deleting an actor

environment.go

@@ -151,7 +151,9 @@ func (this *environment) Del(ctx context.Context, actors ...Actor) error {
 		if info.stopped != nil {
 			channels = append(channels, info.stopped)
 		}
-		info.done()
+		if info.done != nil {
+			info.done()
+		}
 	}
 	for _, channel := range channels {
 		if channel == nil { continue }