TypeError: Cannot read properties of null (reading ‘concat’)

if (ctrl.initialized.MedicalHistoryForms) {
  var clonedList = _.cloneDeep($scope.timelineRecords);
  clonedList = ctrl.resetItemsInTimeLine(ctrl.medicalHistoryForms, clonedList, 'MedicalHx', 'FormAnswersId');
  $scope.timelineRecords = clonedList;
} else {
  $scope.timelineRecords = $scope.timelineRecords.concat(ctrl.medicalHistoryForms);
  ctrl.initialized.MedicalHistoryForms = true;
}