Kivy ScrollView in ScreenManager

Question

I am trying to create several scrollable main screens that are connected with ScreenManager However, when I try, I get the following error:

Only one root object is allowed by .kv

This happens when I add the WindowsManager in the kv file.

Can anyone advice me with how to resolve this?

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import StringProperty
from kivy.uix.screenmanager import ScreenManager, Screen


class WindowManager(ScreenManager):
    pass

class MenuPage(Screen):
    pass

class MainPage(Screen):
    pass



class Sections(BoxLayout):
    label_text = StringProperty()


kv = Builder.load_file('main text')

class Scrollable(App):
    def build(self):
        return kv

    def on_start(self):
        self.root.ids.sv_box.add_widget(Sections(label_text=''))



Scrollable().run()



**kv file**

WindowsManager:
    MainPage:
    MenuPage:
    
BoxLayout:
    orientation: 'vertical'

    ScrollView:
        do_scroll_y: True
        BoxLayout:
            orientation: 'vertical'
            id: sv_box
            size_hint_y: None
            height: self.minimum_height

<Sections>:
    orientation: 'vertical'
    size_hint_y: None
    height: 800
    BoxLayout:
        size_hint: (1,.5)
        Button:
            text: 'Menu'
            size_hint: (.3,1)
    BoxLayout:
        Label:
            text: 'Time'
    ```

Answer 1

According to kivy documentation a .kv file must contain only one root widget at most. In other words you can not have more than one class in the left most indentation level (except for dynamic classes) in kvlang.

In your posted code of .kv file there are two classes (WindowsManager and BoxLayout) at left(most) with same indentation level.

As from your code it's not clear which one of these two is the root widget, I assumed WindowsManager to be the root and moved the BoxLayout stuff under MainPage. Thus your modified main text.kv file now looks like,

WindowManager: # WindowsManager
    MainPage:
        BoxLayout:
            orientation: 'vertical'

            ScrollView:
                do_scroll_y: True
                BoxLayout:
                    orientation: 'vertical'
                    id: sv_box
                    size_hint_y: None
                    height: self.minimum_height

    MenuPage:


<Sections>:
    orientation: 'vertical'
    size_hint_y: None
    height: 800
    BoxLayout:
        size_hint: (1,.5)
        Button:
            text: 'Menu'
            size_hint: (.3,1)
    BoxLayout:
        Label:
            text: 'Time'