prevent crashes in pianoroll when showing velocity

Crash caused by an assumption that all automation has a line

gtk2_ardour/ghost_event.cc

@@ -46,6 +46,13 @@ GhostEvent::~GhostEvent ()
 	/* event is not ours to delete */
 	delete item;
 }
+
+void
+GhostEvent::set_sensitive (bool yn)
+{
+	item->set_ignore_events (!yn);
+}
+
 /** Given a note in our parent region (ie the actual MidiRegionView), find our
  *  representation of it.
  *  @return Our Event, or 0 if not found.

gtk2_ardour/ghost_event.h

@@ -22,6 +22,8 @@ class GhostEvent : public sigc::trackable
 	GhostEvent (::NoteBase *, ArdourCanvas::Container *, ArdourCanvas::Item* i);
 	virtual ~GhostEvent ();
 
+	void set_sensitive (bool yn);
+
 	NoteBase* event;
 	ArdourCanvas::Item* item;
 	bool is_hit;

gtk2_ardour/pianoroll_midi_view.cc

@@ -505,11 +505,19 @@ PianorollMidiView::internal_set_active_automation (Evoral::Parameter const & par
 
 	for (CueAutomationMap::iterator i = automation_map.begin(); i != automation_map.end(); ++i) {
 		if (i->first == param) {
-			i->second.line->set_sensitive (true);
+			if (i->second.line) {
+				/* velocity does not have a line */
+				i->second.line->set_sensitive (true);
+			} else {
+			}
 			active_automation = &i->second;
 			exists = true;
 		} else {
-			i->second.line->set_sensitive (false);
+			if (i->second.line) {
+				i->second.line->set_sensitive (false);
+			} else {
+				i->second.velocity_display->set_sensitive (false);
+			}
 		}
 	}
 
@@ -658,4 +666,3 @@ PianorollMidiView::clear_selection ()
 		i->second.line->set_selected_points (empty);
 	}
 }
-

gtk2_ardour/velocity_display.cc

@@ -156,6 +156,14 @@ VelocityDisplay::clear ()
 	_optimization_iterator = events.end();
 }
 
+void
+VelocityDisplay::set_sensitive (bool yn)
+{
+	for (auto & ev : events) {
+		ev.second->set_sensitive (yn);
+	}
+}
+
 void
 VelocityDisplay::add_note (NoteBase* nb)
 {

gtk2_ardour/velocity_display.h

@@ -65,6 +65,8 @@ class VelocityDisplay
 
 	int y_position_to_velocity (double y) const;
 
+	void set_sensitive (bool yn);
+
 	void set_selected (bool);
 
 	bool line_draw_motion (ArdourCanvas::Duple const & d, ArdourCanvas::Rectangle const & r, double last_x);