Merge pull request #1488 from akka/wip-3393-Trigger-NPE-∂π

do not close over context.self in TickGenerator, see #3393

akka-actor/src/main/scala/akka/io/Pipelines.scala

@@ -1197,10 +1197,13 @@ class TickGenerator[Cmd <: AnyRef, Evt <: AnyRef](interval: FiniteDuration)
     new PipePair[Cmd, Cmd, Evt, Evt] {
 
       // use unique object to avoid double-activation on actor restart
-      private val trigger: Trigger =
+      private val trigger: Trigger = {
+        val path = ctx.getContext.self.path
+
         new Trigger {
-          override def toString = s"Tick[${ctx.getContext.self.path}]"
+          override def toString = s"Tick[$path]"
         }
+      }
 
       private def schedule() =
         ctx.getContext.system.scheduler.scheduleOnce(