use NodeInputArray for tracks as well

These were introduced for the new Block system and make just as much sense for
the Track system. They've now been implemented for both.

app/node/output/timeline/timeline.cpp

@@ -34,7 +34,7 @@ TimelineOutput::TimelineOutput()
 
   for (int i=0;i<kTrackTypeCount;i++) {
     // Create track input
-    NodeInput* track_input = new NodeInput(QString("track_in_%1").arg(i));
+    NodeInputArray* track_input = new NodeInputArray(QStringLiteral("track_in_%1").arg(i));
     AddInput(track_input);
     track_inputs_.replace(i, track_input);
 
@@ -106,7 +106,13 @@ void TimelineOutput::UpdateTrackCache()
   track_cache_.clear();
 
   foreach (TrackList* list, track_lists_) {
-    track_cache_.append(list->Tracks());
+    QVector<TrackOutput*> track_list = list->Tracks();
+
+    foreach (TrackOutput* track, track_list) {
+      if (track) {
+        track_cache_.append(list->Tracks());
+      }
+    }
   }
 }