Improve barcode attribute handling and enhance error logging in TemplateManager

- Updated barcode attributes to provide default values when content or attributes are missing, ensuring more robust barcode generation.
- Added error logging in the preview method to capture and log errors more effectively, improving debugging capabilities.

src/templates/templatemanager.js

@@ -179,9 +179,9 @@ async function transformCustomElements(content) {
               attrs: {
                 class: 'documentBarcode',
                 'jsbarcode-displayValue': 'false',
-                'jsbarcode-value': node.content[0],
+                'jsbarcode-value': node.content.at(0) || '',
-                'jsbarcode-format': node.attrs.format,
+                'jsbarcode-format': node.attrs.format || 'code128',
-                'jsbarcode-width': node.attrs.barcodeWidth,
+                'jsbarcode-width': node.attrs.barcodeWidth || 2,
                 'jsbarcode-margin': 0
               }
             }