TypeError：必须是str，而不是NoneType ..构建计算器

Question

# Calculator.py
from graphics import *
from button import Button

class Calculator:
    def __init__(self):
        win = GraphWin("Calculator")
        win.setCoords(0,0,6,7)
        win.setBackground("slategray")
        self.win = win

        self.__createButtons()
        self.__createDisplay()

    def __createButtons(self):
        bSpecs = [(2,1,"0"), (3,1,"."),
                  (1,2,"1"), (2,2,"2"), (3,2,"3"), (4,2,"+"), (5,2,"-"),
                  (1,3,"4"), (2,3,"5"), (3,3,"6"), (4,3,"*"), (5,3,"/"),
                  (1,4,"7"), (2,4,"8"), (3,4,"9"), (4,4,"<-"), (5,4,"C")]
        self.buttons = []

        for (cx, cy, label) in bSpecs:
            self.buttons.append(Button(self.win,Point(cx,cy),
                                       .75,.75,label))

        self.buttons.append(Button(self.win, Point(4.5,1),
                                   1.75, .75, "="))

        for b in self.buttons:
            b.activate()

    def __createDisplay(self):
        bg = Rectangle(Point(.5, 5.5), Point(5.5,6.5))
        bg.setFill("white")
        bg.draw(self.win)
        text = Text(Point(3,6), "")
        text.draw(self.win)
        text.setFace("courier")
        text.setStyle("bold")
        text.setSize(16)
        self.display = text



    def getButton(self):
        while True:
            p = self.win.getMouse()
            for b in self.buttons:
                if b.clicked(p):
                    return b.getLabel()

    def processButton(self, key):
        text = self.display.getText()
        if key == "C":
            self.display.setText("")
        elif key == "<-":
            self.display.setText(text[:-1])
        elif key == "=":
            try:
                result = eval(text)
            except:
                result = "ERROR"
            self.display.setText(str(result))
        else:
            self.display.setText(text+key)

    def run(self):
        while True:
            key = self.getButton()
            self.processButton(key)

if __name__ == "__main__":
    theCalc = Calculator()
    theCalc.run()

嗨，我是Python的新手，我试图创建这个计算器，但我一直收到错误消息：

line 74, in <module>
    theCalc.run()
line 70, in run
    self.processButton(key)
line 65, in processButton
    self.display.setText(text+key)
TypeError: must be str, not NoneType

如果有人能向我解释我做错了什么，我会很高兴。