StringIndexOutOfBoundsException when click on a hyperlink with empty reference address (such as ZSS-457).

java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(String.java:658)
at org.zkoss.poi.ss.util.CellReference.separateRefParts(CellReference.java:355)
at org.zkoss.poi.ss.util.CellReference.<init>(CellReference.java:93)
at org.zkoss.poi.ss.util.AreaReference.<init>(AreaReference.java:60)
at org.zkoss.poi.ss.util.AreaReference.<init>(AreaReference.java:44)
at org.zkoss.zss.ui.Spreadsheet.service(Spreadsheet.java:5972)
at ...