Google Apps Script withSuccessHandler() gives back always null

I’m using Google Apps Script withSuccessHandler() and even if the logs from Logger are all correct, after returning from the function in the backend, the result is always a null.

Here’s reported the code block where I call the google.script.run.withSuccessHandler() referring to the function checkBarcode().

// Check if the vessel exists
      function checkVessel() {
        const barcode = document.getElementById("barcode").value.trim();
        barcode.toString();
        if (!barcode) {
          alert("Please scan or enter a barcode.");
          return;
        }
        
        console.log("Sending barcode to server:", barcode); // Debugging
        google.script.run.withSuccessHandler(result => {
          console.log("Result received from server:", result); // Debugging
          showForm(result);;
        }).checkBarcode(barcode)

      }

Here’s reported the function checkBarcode (also the function findBarcode used in checkBarcode).

// Check if a vessel with the barcode exists
function checkBarcode(barcode) {
  barcode.toString().trim();
  Logger.log("Checking barcode: " + barcode);

  // Search for barcode in N2 sheet
  const n2Result = findBarcode("N2", barcode);

  if (n2Result) {
    Logger.log("Barcode found in N2.");
    return result = {
      sheet: "N2",
      row: n2Result.row,
      data: n2Result.data,
    };
  }

  // Search for barcode in N2O sheet
  const n2oResult = findBarcode("N2O", barcode);

  if (n2oResult) {
    Logger.log("Barcode found in N2O.");
    return result = {
      sheet: "N2O",
      row: n2oResult.row,
      data: n2oResult.data,
    };
  }

  // If not found in any sheet
  Logger.log("Barcode not found in any sheet.");
  return result = null; // Return null explicitly if not found
}

// Find barcode in a sheet
function findBarcode(sheetName, barcode) {
  const spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = spreadsheet.getSheetByName(sheetName);

  if (!sheet) {
    Logger.log("Sheet not found: " + sheetName);
    return null;
  }

  const data = sheet.getDataRange().getValues();
  Logger.log("Data in sheet " + sheetName + ": " + JSON.stringify(data));

  for (let i = 0; i < data.length; i++) {
    Logger.log("Comparing: " + data[i][0].toString().trim() + " with " + barcode.toString().trim());
    if (data[i][0].toString().trim() === barcode.toString().trim()) {
      return { row: i + 1, data: data[i] }; // Return row index and data
    }
  }
  return null; // Not found
}

I tried to avoid using multiple function in the .gs file but the results were the same. I also tried to convert the barcode variable in string (probably too many times) to avoid errors.