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What was printed out when you added a println() statement to print out the values of the variables used on line 41? Which variable on that line has a null value? To solve this kind of error you need...

Print the values of all the variables used on line 41 to see which one has the null value.

What variable has the null value? Why doesn't that variable have a valid value?

Please copy the full text of the error message and post it here.

What variable has the null value? Backtrack in the code to see why that variable does not have a valid value.