open_spreadsheet() returns invalid SpreadsheetDocument for a locked file

If I open e.g. "1.ods" in LibreOffice through mouse clicking then run
```
desktop = pyoo.Desktop(pipe='libreoffice')
doc = desktop.open_spreadsheet('1.ods')
```
under the hood loadComponentFromURL() returns None and then SpreadsheetDocument(None) is returned from open_spreadsheet().

PS Thanks for the lib, LibreOffice API is scary :)