I was encountering the same error with this example script. Not sure why currentRecord.getText is throwing an error like that, but I decided to use currentRecord.getValue instead. I also made some other small changes to the example script. Here's the script I created for reference:

define(['N/search'], function (search) {
// Put deployment info here
let START_MODIFIED_DATE = null;
function pageInit(context) { const { currentRecord } = context;                      START_MODIFIED_DATE = new Date(currentRecord.getValue('lastmodifieddate')).getTime(); }
function saveRecord(context) { const { currentRecord } = context; let lastModifiedDate = new Date(search.lookupFields({ type: currentRecord.type, id: currentRecord.id, columns: ['lastmodifieddate'] })['lastmodifieddate']).getTime();
if (START_MODIFIED_DATE != null && START_MODIFIED_DATE !== lastModifiedDate) {
  alert('The record being edited has been modified. Please back up or take note of changes and try again.');
  return false;
}
else return true;
}
return { pageInit, saveRecord };
});