Kivy Add Dropdown Programmtically to existing layout - kivy

Question

I would like to add dropdown to a layout that I already have but I'm having issues when I add it. the layout randomly displays the options but not the mainbutton. Is there something am I adding the incorrect widget?
NOTE: I prefer to create my layouts dynamically, so I am not using the kv files. All of the examples I have seen just use the runapp function instead of adding the dropdown to an existing layout.
My Code:
Window.size = (Config.getint('graphics','width'),Config.getint('graphics','height'))
Window.top = 30
Window.left = 10
screen_width = 700
screen_height = 775
Window.clearcolor = color_palette["Royal Purple"]
Window.size = (screen_width,screen_height)
Config.write()
##~ dropdown function ~##
def createDropDown():
dropdown = DropDown()
categories = ['Cat1','Cat2','Cat3','Cat4']
for index in range(len(categories)):
btn = Button(text=categories[index], size_hint_y=None, height=44)
btn.bind(on_release=lambda btn: dropdown.select(btn.text))
dropdown.add_widget(btn)
mainbutton = Button(text='Hello', size_hint=(None, None))
mainbutton.bind(on_release=dropdown.open)
dropdown.bind(on_select=lambda instance, x: setattr(mainbutton, 'text', x))
return dropdown
##~~ My main layout class ~~##
class SimpleKivy0(App):
App.title = "New GUI"
btn_width = 0.2
btn_height = 0.05
txt_fld1 = None
entry_fld_Label = None
def build(self):
layout = FloatLayout()
banner_bground = layout.canvas.before
with banner_bground:
Rectangle(pos=(0,screen_height-(screen_height*0.1)),size=(screen_width, screen_height*0.1))
Color(68/255,210/255,201/255,1)
banner_Lbl = Label(text='Quick Query GUI',halign='left',valign='middle',font_size=((screen_height*0.1)*0.7),pos=(0,screen_height-(screen_height*0.1)),size_hint=(1,0.1))
banner_Lbl.bind(size=banner_Lbl.setter('text_size'))
layout.canvas.add(banner_bground)
layout.add_widget(banner_Lbl)
dropdown = createDropDown()
dropdown.size_hint=(.25,.1)
dropdown.pos = (screen_width*0.2,screen_height*0.2)
return layout
if __name__ == "__main__":
SimpleKivy0().run()
The screenshot:

Answer 1

The main thing you were missing is to add the mainbutton to your layout. Here is a working version of some of your code:
def createDropDown(layout):
dropdown = DropDown()
categories = ['Cat1','Cat2','Cat3','Cat4']
for index in range(len(categories)):
btn = Button(text=categories[index], size_hint=(None, None), size=(100,44))
btn.bind(on_release=lambda btn: dropdown.select(btn.text))
dropdown.add_widget(btn)
mainbutton = Button(text='Hello', size_hint=(None, None), size=(100,40), pos=(100,100))
mainbutton.bind(on_release=dropdown.open)
dropdown.bind(on_select=lambda instance, x: setattr(mainbutton, 'text', x))
# Add mainbutton to the layout
layout.add_widget(mainbutton)
##~~ My main layout class ~~##
class SimpleKivy0(App):
App.title = "New GUI"
btn_width = 0.2
btn_height = 0.05
txt_fld1 = None
entry_fld_Label = None
def build(self):
layout = FloatLayout()
banner_bground = layout.canvas.before
with banner_bground:
Rectangle(pos=(0,screen_height-(screen_height*0.1)),size=(screen_width, screen_height*0.1))
Color(68/255,210/255,201/255,1)
banner_Lbl = Label(text='Quick Query GUI',halign='left',valign='middle',font_size=((screen_height*0.1)*0.7),pos=(0,screen_height-(screen_height*0.1)),size_hint=(1,0.1))
banner_Lbl.bind(size=banner_Lbl.setter('text_size'))
layout.canvas.add(banner_bground)
layout.add_widget(banner_Lbl)
# create the dropdown and add the mainbutton to the layout
createDropDown(layout)
return layout

Answer 2

For those who are also having an issue with this, I came across an alternative called a spinner. The spinner object worked the same as I intended for the popup.
This is the code I added instead of the dropdown:
def get_spinner_selection(spinner,text):
print(text)
spinner = Spinner(text='Option1',values=('Option1','Option2','Option3'),size_hint=(0.2,0.1),background_color=(169/255,80/255,18/255,1))
spinner.bind(text=get_spinner_selection)
layout.add_widget(spinner)
queryType_Lbl = Label(text='Query Type:',valign='middle',size_hint=self.spinner.size_hint,pos=(spinner.x,spinner.top))