fix null error in calculateWordRepetitionInstruction

src/MusicalScore/Graphical/VexFlow/VexFlowMusicSheetCalculator.ts

@@ -791,7 +791,7 @@ export class VexFlowMusicSheetCalculator extends MusicSheetCalculator {
     const measures: VexFlowMeasure[] = <VexFlowMeasure[]>this.graphicalMusicSheet.MeasureList[measureIndex];
     for (let idx: number = 0, len: number = measures.length; idx < len; ++idx) {
       const graphicalMeasure: VexFlowMeasure = measures[idx];
-      if (graphicalMeasure.ParentStaffLine && graphicalMeasure.ParentStaff.ParentInstrument.Visible) {
+      if (graphicalMeasure && graphicalMeasure.ParentStaffLine && graphicalMeasure.ParentStaff.ParentInstrument.Visible) {
         uppermostMeasure = <VexFlowMeasure>graphicalMeasure;
         break;
       }