failed to show data validation dialog

Steps to Reproduce

1. import the attached file
2. right-click A1, click "Data validation" in the context menu to show the dialog

Current Result

java.lang.NullPointerException
at io.keikai.model.CellRegion.overlaps0(CellRegion.java:125)
at io.keikai.model.CellRegion.overlaps(CellRegion.java:121)
at io.keikai.model.CellRegion.diff(CellRegion.java:221)
at io.keikaiex.ui.impl.ua.DataValidationHandler.isOverlapped(DataValidationHandler.java:126)
at io.keikaiex.ui.impl.ua.DataValidationHandler.processAction(DataValidationHandler.java:66)
at io.keikai.ui.impl.ua.AbstractHandler.process(AbstractHandler.java:63)
at io.keikai.ui.impl.DefaultUserActionManagerCtrl.dispatchAuxAction(DefaultUserActionManagerCtrl.java:354)
at io.keikai.ui.impl.DefaultUserActionManagerCtrl.onEvent(DefaultUserActionManagerCtrl.java:513)
at io.keikaiex.ui.impl.UserActionManagerCtrlImpl.onEvent(UserActionManagerCtrlImpl.java:324)
at io.keikai.ui.Spreadsheet$4.onEvent(Spreadsheet.java:541)

Expected Result

show the data validation on the cell

Debug Information

Workaround

CellRegion.java