[Tkinter] Close Toplevel after clicking button

[jollydragon]

After runing below code, got below error.
How to colse the Toplevel window after clicking the button in my Python v3.6.5?
Thanks for any help or suggestion.

Error:
_tkinter.TclError: bad window path name ".!toplevel"

def mdfNames(mdf):
     mdf.destroy()
     

def mdfPanel():
     mdf = tkinter.Toplevel()
     mdfSize = 220
     mdf.geometry('%dx%d+%d+%d' % (mdfSize, mdfSize, (mdf.winfo_screenwidth()-mdfSize)/2,
                    (mdf.winfo_screenheight()-mdfSize)/2))                

     tkinter.Button(mdf, text = "Start", command=mdfNames(mdf), justify = tkinter.CENTER).grid(row=7,column =1, columnspan =2)