Error: The getter 'spot' was called on null about fl_chart HOT 2 CLOSED

touchTooltipData: TouchTooltipData(
                          tooltipBgColor: Colors.blueGrey,
                          getTooltipItems: (touchedSpots) {
                            return touchedSpots.map((touchedSpot) {
                              if (touchedSpot?.spot == null) { return null; }  // ***** this is the key code
                              return TooltipItem('***' + '\n' + touchedSpot.spot.y.toString(), TextStyle(color: Colors.yellow));
                            }).toList();
                          }
                        )