kivy CheckBox Get Value in other class

right now i try to learn a little bit of python and kivy.

i have a small problem of how to get the value of a CheckBox (True or False) in another Class. Within the class its not a problem: Python Code:

class hauptmenue(Screen, Widget):
   pruefung = ObjectProperty(None)
   grundlagen = ObjectProperty(None)
   def checkbox_clicked_1(self, instance, value):
       if self.pruefung.active == True:
           self.grundlagen.active = True
       if value is True:
       [...]

This works just fine. I the kv. Code I did that:

       GridLayout:
           cols:4
           size_hint_y: 0.1              
           NeuLabel:
               text: "Grundlagen"                
           NeuCheckBox:
               id: grundlagen
               on_active: root.checkbox_clicked_1(self, self.active)

now for the Problem: how to get hat value (True/false) in the other class?

class Auswertung_Ende(Screen):
   def on_enter(self):
       if hauptmenue.pruefung.active == True:
           print("Hello")

The error ist: AttributeError: 'kivy.properties.ObjectProperty' object has no attribute 'active'

Why does it work in the same class but not in another? I just want to get the value (True/False) of the CheckBox from another class (here:Screen). That's all... I am sorry if this question is on a very low level. I am still learning :)

Many thanks!