fix undefined error with drawUpToMeasureNumber set (#68)

src/OpenSheetMusicDisplay/Cursor.ts

@@ -247,7 +247,7 @@ export class Cursor implements IPlaybackListener {
     }
 
     // y is common for both multirest and non-multirest, given the MusicSystem
-    y = musicSystem.PositionAndShape.AbsolutePosition.y + musicSystem.StaffLines[0].PositionAndShape.RelativePosition.y ?? 0;
+    y = musicSystem.PositionAndShape.AbsolutePosition.y + musicSystem.StaffLines[0]?.PositionAndShape.RelativePosition.y ?? 0;
     const bottomStaffline: StaffLine = musicSystem.StaffLines[musicSystem.StaffLines.length - 1];
     const endY: number = musicSystem.PositionAndShape.AbsolutePosition.y +
     bottomStaffline.PositionAndShape.RelativePosition.y + bottomStaffline.StaffHeight;