from tkinter import *
#from tkinter.messagebox import showerror
class calcsheet:
	def __init__(self):
		self.frame = Frame(window, bg="blue", bd=20)
		self.frame.pack()
		self.thickness = self.rowentry(0, 'Толщина')
		self.lenght = self.rowentry(1, 'Длина')
		self.width = self.rowentry(2, 'Ширина')
		self.amount = self.rowentry(3, 'Кол-во')
	def rowentry(self, row, text):
		self.lab = Label(self.frame, text=text)
		self.entry = Entry(self.frame, width=20)
		self.lab.grid(row=row,column=0)
		self.entry.grid(row=row,column=1, padx=10)
def printer(event):
	print(s.thickness.entry.get())
window = Tk()
window.title('Person')
s = calcsheet()
window.bind('<Return>', printer)
window.mainloop()

print(s.thickness.entry.get())
AttributeError: 'NoneType' object has no attribute 'entry'

print(s.entry.get())