stickyRowsOrColumns feature : 1. It calls `findElementInsideElement(xml, 'pane', sheetViewElement)` to find a `pane` element inside 2. `findElementInsideElement` calls `findElement(getMarkupInsideElement(xml, sheetViewElement), 'pane')` 3. Since no `pane` element exists yet, `findElement` returns `undefined` 4. Then `applyEnclosingElementOffset(undefined, sheetViewElement)` is called 5. **Bug**: `applyEnclosingElementOffset` accesses `element.openingTagStartIndex += ...` **without checking if element is undefined first**! FIX applyEnclosingElementOffset : ```javascript export default function applyEnclosingElementOffset(element, enclosingElement) { + if (!element) return; var enclosingElementOffset = enclosingElement.openingTagEndIndex + 1; element.openingTagStartIndex += enclosingElementOffset; ... ```