How do I access the first jframe while I'm in the second jframe?

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Java

Let's say I run the first JFrame window and then from the 1st frame I open the 2nd frame. 1st frame is still open and now I want to access the first frame from the 2nd frame but I get NullPointerException if I use any methods or Objects from the fist frame. Even I looked online but all the videos are only talking about how to access 2nd frame from 1st frame. Please help me on this.
Thanks.

What I have tried:

In the first frame:
//constructor
class2 c = new class2();
//constructor2
method1();

//method1();
//main class

In the second frame:
//constructor
class1 = new class1();//runs the constructor2
//Here I get an NullPointerException error