steps to reproduce

1. load a blank sheet
2. calling Ranges.range(ss.getBook()).createSheet("temp"); or SheetOperationUtil.createSheet(Ranges.range(ss.getBook()),"temp");

result

SEVERE:
java.lang.NullPointerException
at org.zkoss.zss.api.impl.RangeImpl.getBook(RangeImpl.java:317)
at org.zkoss.zss.api.SheetOperationUtil$2.run(SheetOperationUtil.java:348)
at org.zkoss.zss.api.impl.RangeImpl.sync(RangeImpl.java:263)
at org.zkoss.zss.api.SheetOperationUtil.createSheet(SheetOperationUtil.java:346)

expected result

no exception
Passing a book range is also a reasonable range.