Question: the code for the total is not working it gives me an error that says cannot invoke java.lang.Double.doubleValue() because the return value of java.

the code for the total is not working it gives me an error that says " cannot invoke java.lang.Double.doubleValue()" because the return value of java. swing.table.DefaultTableModel.getValueAt(int,int) is null.
my problem is that i want to add the price each time the user add book to the jtable. how should i fix it?
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