fix GetNearestVoiceEntry throwing in rare cases (add null check) (#8)

fixes #8

src/MusicalScore/Graphical/GraphicalMusicSheet.ts

@@ -570,7 +570,7 @@ export class GraphicalMusicSheet {
     public GetNearestVoiceEntry(clickPosition: PointF2D): GraphicalVoiceEntry {
         return this.GetNearestGraphicalObject<GraphicalVoiceEntry>(clickPosition, GraphicalVoiceEntry.name, 5, 20, 5,
                                                                    (object: GraphicalVoiceEntry) =>
-                                                                        object.parentStaffEntry.relInMeasureTimestamp !== undefined);
+                                                                        object.parentStaffEntry?.relInMeasureTimestamp !== undefined);
     }
 
     public GetNearestNote(clickPosition: PointF2D, maxClickDist: PointF2D): GraphicalNote {