Excel Export fails with exception

Reproduced on nightly 20141030

See exception in attachment.

Steps to reproduce problem:
1. Create folder with 2 File children and at least 6 duplicate Note children
2. Navigate to above folder
3. Click 'Spreadsheet' Action
4. Close 'Spreadsheet' dialog by clicking 'X' in upper right corner
5. Click 'Excel Export' Action

Note error is reproduced.

See URL https://nightly.nuxeo.com/nuxeo/nxdoc/default/b3976224-2fab-4e88-8f54-15883c162086/view_documents on current nightly build as of 20141103 as example.