Change Text Input field from Screen when i press a button in a popup window Python Kivy

Hello I am try to make a type of dropdown selection for my text input using a popup window but it doesn't work .

Python code:

class SecondWindow(Screen):
    def __init__(self,**kwargs):
        super(SecondWindow, self).__init__()
    txt_inpt = ObjectProperty()
    add =ObjectProperty()
    finish = ObjectProperty()
    numar_bucati = ObjectProperty()
    tip_schela = ObjectProperty()
    metri_schela = ObjectProperty()
    tip_motor = ObjectProperty()
    numar_motoare = ObjectProperty()

    def re_enter(self):
        with open("popupid/light_selected","r") as popi:
            popir= popi.read()
        if popir != "" :
            self.ids.txt_inpt.text = popir
        else:
            pass

    def lights_drop(self,instance):
        if instance in self.ids.values():
            popup_id=list(self.ids.keys())[list(self.ids.values()).index(instance)]
            with open("popupid/popup_id.txt","w") as poptxt:
                poptxt.write(popup_id)
                poptxt.close()
        LightsPop().open()

    def selected_item(self,instance):
        if instance.text in aparate.keys():
            with open("popupid/light_selected", "w") as lighsel:
                lighsel.write(instance.text)
            self.re_enter()
        else:
            pass


    def on_open(self):
        with open('popupid/popup_id.txt','r') as popread:
            mdx = popread.read()

        if mdx == "droplightbutton" :
            for x in fixture_list.split("\n"):
                b= Button(text=x,font_size=14,size_hint=(1,0.8))
                b.bind(on_press=SecondWindow().selected_item)
                self.ids.drop2.add_widget(b)

Kivy:

TextInput:
                id: txt_inpt
                text: ""

                font_size:14
                multiline:False
                size_hint:(0.7,1)
                hint_text:"type your fixture here"

Button:
                id:droplightbutton
                y: self.parent.y
                x: self.parent.x
                text: ""
                markup: True
                size_hint:(0.2,1)
                background_normal: ''
                background_color: (0,0,0,0)
                on_press: root.lights_drop(self)
                Image:
                    source: 'images/drop_down2.png'
                    y: self.parent.y
                    x: self.parent.x
                    size_hint:(None,None)
                    size: (40,40)