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I solved this problem thanks for a tip of Edouard Renard (Udemy: ROS for Beginners Course). I implemented a singleton in each class and I did not use inheritance, but composition. This helped me to solve the problem. The code was something like this:

class db_connect(object):
     # Some stuff to connect with DB

class ROSController(objetc):
       # singleton flag
    _instance = None

    @classmethod
    def instance(cls):
        if cls._instance is None:
            cls._instance = cls()
        return cls._instance

    def __init__(self, parent=None):
        super(ROSController, self).__init__(parent)
        ...

class MainController(db_connect):
    def __init__(self):
        super(MainController, self).__init__()
        self.ros_node = RosController.instance()

class Window_1(object):
    # Main window containing buttons and other things.
    def __init__(self):
        super(Window_1, self).__init__()
        self.setupUi(self)
        self.ros_node = RosController.instance()