setSelectedSheet() causes a NullPointerException

Steps to Reproduce

1. run the attached zul and composer

<spreadsheet id="ss" width="100%" vflex="1" apply="ticket.HideSelectedComposer"
showContextMenu="true" showSheetbar="true" showToolbar="true" showFormulabar="true"
showSheetTabContextMenu="true"
/>

Current Result

java.lang.NullPointerException
at io.keikai.ui.Spreadsheet.setSelectedSheet0(Spreadsheet.java:1034)
at io.keikai.ui.Spreadsheet.setSelectedSheet(Spreadsheet.java:983)
at ticket.HideSelectedComposer.doAfterCompose(HideSelectedComposer.java:27)

_initializedSheets is null

Expected Result

no error.
Select the first visible sheet

Debug Information

• the selected sheet in a file is hidden by API, which is the real use case from a customer
• we could initialize _initializedSheets early