I dont understand this problem and i need to do this for my project,

Question

My project is about a simple "book" library in java and I need to do this "check in check out" class

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Properties.getProperty(String)" because the return value of "org.jdatepicker.impl.JDatePanelImpl.access$500(org.jdatepicker.impl.JDatePanelImpl)" is null
    at org.jdatepicker.impl.JDatePanelImpl$InternalView.getPreviousMonthButton(JDatePanelImpl.java:517)
    at org.jdatepicker.impl.JDatePanelImpl$InternalView.getPreviousButtonPanel(JDatePanelImpl.java:442)
    at org.jdatepicker.impl.JDatePanelImpl$InternalView.getNorthPanel(JDatePanelImpl.java:251)
    at org.jdatepicker.impl.JDatePanelImpl$InternalView.initialise(JDatePanelImpl.java:234)
    at org.jdatepicker.impl.JDatePanelImpl$InternalView.<init>(JDatePanelImpl.java:222)
    at org.jdatepicker.impl.JDatePanelImpl.<init>(JDatePanelImpl.java:110)
    at CheckInCheckOut.main(CheckInCheckOut.java:22)

I really don't know what to do , please help