Fatal Error on non object in Worksheet.php

the disconnectCells method is not always fully initialized with the attribute 
_cellCollection, so a check needs to be done. Otherwise a fatal error occurs.
happens if multiple xsl are created from csv file
changed NULL to null
Bugfix: Work item GH-80 - "Sheet index is out of bounds." Exception
Example of cell-level protection in a worksheet
Merge branch 'develop' into calcEngine