Welcome to the Ranch
I suggest you get rid of all those
= null assignments. They are redundant. Make sure all those references are initialised in your constructor (or a private or final initGUI or similar method called from the constructor). It may be unnecessary to have that many components as fields. Also get rid of commented-out stuff from previous applications. Leaving old code in might make it easier to calculate Fahrenheit, but when you are trying to work out a mortgage, such old code is a potential source of errors.
I suggest you need to learn to write an anonymous class. I have written about them often enough: try
here.