TypeError: super(type, obj) when using exec()

Below is the whole (test) app written with Kivy. This is something like app preview application: user enters a text of kv markup (see variable self.kv) and a text of classes (see variable self.text). Then he clicks the "preview" button and sees the result on the right side of the application. Loading kv is implemented using kivy Builder.load_string(). Class loading is implemented using exec(, globals()).

The main problem is that for some reason I get the following error when I click on the preview button for the third time (the first 2 clicks work without errors):

TypeError: super(type, obj): obj must be an instance or subtype of type

The error can be because of exec(), (without exec I don’t get this error).

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button

KV = '''
BoxLayout:
    BoxLayout:
        orientation: 'vertical'

        CodeEd
            id: code_editor

        Button:
            text: 'Preview'
            on_release: app.preview()

    Preview:
        id: preview_area

<CodeEd@TextInput>
    text: app.text

<Preview@RelativeLayout>
'''

class MyApp(App):
    def build(self):
        self._kv_filename = 'KvEditor_internal.' + str(self.uid)

        self.text = '''
class MyButton(Button):
    def on_touch_down(self, touch):
        if self.collide_point(*touch.pos):
            print (333)
        super(MyButton, self).on_touch_down(touch)


'''
        self.kv = 'MyButton'

        self.root = Builder.load_string(KV)


    def preview(self):
        preview_area = self.root.ids.preview_area

        #if 'MyButton' in globals():
        #    del globals()['MyButton']

        #print ('===================')
        #print ([i for i in dict(globals())])

        try:
            exec(self.text, globals())
        except:
            print ('some error when exec class ')

        Builder.unload_file(self._kv_filename)

        try:

            preview_area.add_widget(Builder.load_string(self.kv, filename=self._kv_filename))

        except Exception as e:
            print (e.message if getattr(e, r"message", None) else str(e))


MyApp().run()

How to solve this problem?

from TypeError: super(type, obj) when using exec()