KivyMD: DropdownMenu callback stopped working

Description of the Bug

The DropdownMenu callback is not invoked. This worked before the latest MDMenu changes.

Code and Logs

Below is the example from the docs, modified with adding a callback https://kivymd.readthedocs.io/en/latest/components/menu/index.html#usage

The text “======selected======” does not appear in the console

from kivy.lang import Builder

from kivymd.app import MDApp
from kivymd.uix.menu import MDDropdownMenu

KV = '''
Screen:

    MDRaisedButton:
        id: button
        text: "PRESS ME"
        pos_hint: {"center_x": .5, "center_y": .5}
        on_release: app.menu.open()
'''


class Test(MDApp):
    def selected(self):
        print("=====selected=======")
    
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.screen = Builder.load_string(KV)
        menu_items = [{"icon": "git",
                       "text": f"Item {i}",
                       "callback": self.selected} for i in range(5)]
        self.menu = MDDropdownMenu(
            caller=self.screen.ids.button, items=menu_items, width_mult=4
        )

    def build(self):
        return self.screen


Test().run()

### Screenshots

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### Versions

* OS:  Windows 10
* Python:  Python 3.6.1
* Kivy:   1.11.1
* KivyMD:   master 4/24/2020