beginner wanting to become intermediate resulting in advance level

Hey I compiled and run a programmed that should display a text field near the top of the frame that iv'e designed already, but its not showing the text field. It gives me the following error:

Exception in thread "main" java.lang.NullPointerException
at Calculator.<init>(Calculator.java:31)
at Calculator.main(Calculator.java:43)

What does this error mean to the code i have coded thus far to create a basic calculator? Please help?

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