Dear John,
thank you for your answer! As I wrote in my first post, I have already tried it. So I change again to the previous version of code:
and I can see the same what previously:
cannot find symbol
symbol: method getContentPane()
location: class NameOfMyClass
The NetBeans
IDE can create this symbol for me, but it still needs to be implemented (and I think I don't need to implement it but rather to change something in the code quoted above):
Regards!