java.lang.NullPointerException error confusion

[bigredaltoid]

Hi all, first post here. I am in a college programming course and have hit a slight roadblock. Attached you will see StudentDriver, Student, and Course. StudentDriver was code that the professor wrote that we are not supposed to edit in any way. Student and Course were provided to us and they had many unimplemented methods that needed to be implemented. I can not figure out why I am getting this error:

Exception in thread "main" java.lang.NullPointerException
at Student.calculateGpa(Student.java:37)
at Student.merge(Student.java:139)
at StudentDriver.main(StudentDriver.java:18)

Everything compiles fine. When I run the StudentDriver, it prompts me to "Please enter the file with the courses taken:", which works fine, and than "Please enter the file with the current courses.", which also seems to be work, but after I enter both files that error pops up.

Thanks in advance.