Steps to Reproduce

1. load the file
2. select color filter on the 3rd column

Current Result

null pointer exception

SEVERE:
java.lang.NullPointerException
at org.zkoss.zss.range.impl.AutoFilterHelper._match(AutoFilterHelper.java:629)
at org.zkoss.zss.range.impl.AutoFilterHelper._filterByColor(AutoFilterHelper.java:655)
at org.zkoss.zss.range.impl.AutoFilterHelper.enableAutoFilter0(AutoFilterHelper.java:799)
at org.zkoss.zss.range.impl.AutoFilterHelper.enableAutoFilter(AutoFilterHelper.java:165)
at org.zkoss.zss.range.impl.RangeImpl$45.invoke(RangeImpl.java:1649)
at org.zkoss.zss.model.util.ReadWriteTask.doInWriteLock(ReadWriteTask.java:43)
at org.zkoss.zss.range.impl.RangeImpl.enableAutoFilter(RangeImpl.java:1654)
at org.zkoss.zss.ui.au.in.AutoFilterDefaultHandler.applyFilter(AutoFilterDefaultHandler.java:712)
at org.zkoss.zss.ui.au.in.FilterCommand.process(FilterCommand.java:49)
at org.zkoss.zss.ui.Spreadsheet.service(Spreadsheet.java:6161)

Expected Result

color filter is disabled (in Excel), since those cells are all blank