generateImages: fix undefined error with playbackmanager.removeListener when playbackmanager not defined

src/OpenSheetMusicDisplay/OpenSheetMusicDisplay.ts

@@ -905,7 +905,7 @@ export class OpenSheetMusicDisplay {
                 // create new cursor
                 if (backendToDrawOn && backendToDrawOn.getRenderElement()) {
                     if (this.cursors[i]) {
-                        this.PlaybackManager.removeListener(this.cursors[i]);
+                        this.PlaybackManager?.removeListener(this.cursors[i]);
                         this.cursors[i].Dispose();
                     }
                     this.cursors[i] = new Cursor(backendToDrawOn.getRenderElement(), this, this.cursorsOptions[i]);