from tkinter import *

class Application(Frame):
    def converter(self):
        try:
            fl1 = float(self.entry1.get())
            fl2 = float(self.entry2.get())
            self.label1["text"] = str(fl1 + fl2)
        except ValueError:
            self.label1["text"] = "***ERROR***"
            
    def createWidgets(self):
        self.button1 = Button(self, text="Convert", command=self.converter)
        self.entry1 = Entry(self)
        self.entry2 = Entry(self)
        self.label1 = Label(self, text = " ")
        self.button1.pack()
        self.entry1.pack()
        self.entry2.pack()
        self.label1.pack()

    def __init__(self, master=None):
        Frame.__init__(self, master)
        self.pack()
        self.createWidgets()

root = Tk()
main = Application(master = root)
main.mainloop()