kivy custom widget bind error

问题

I am trying to make a small tetris game for learning python with kivy. I am trying to create a custom widget with size 20,20. When I add it to the float layout and run the below code I receive the following error:

Error:

File "D:\OS Files\workspace\Tetris\holder.py", line 10, in __init__ self.add_widget(c)
File "C:\Kivy180\kivy\kivy\uix\floatlayout.py", line 115, in add_widget pos_hint=self._trigger_layout)
TypeError: descriptor 'bind' of 'kivy._event.EventDispatcher' object needs an argument

Code: holder.py File:

from items import Cell

class Holder(FloatLayout):
    def __init__(self, **kwargs):
        super(Holder,self).__init__(**kwargs)
        self.size=(300,300)
        c=Cell
        #c.pos= (20,20)
        self.add_widget(c)
        #self.add_widget(c)

items.py File:

from kivy.uix.widget import Widget
from kivy.graphics import *

class Cell(Widget):
    def __init__(self, **kwargs):
        super(Cell,self).__init__(**kwargs)
        with self.canvas:
            Color(1, 0, 0)
            Rectangle(pos=(0, 0), size=(50, 50))

        self.height=50
        self.width=50

main.py File:

from kivy.app import App
from holder import Holder

class start(App):
    def build(self):
        return Holder()

if __name__ == '__main__':
    start().run()

Could you please explain where I went wrong, I am stuck at the starting point itself. Regarding the error, I haven't written any events also, and it is just a widget class. Could you please explain where I went wrong in understanding kivy.