GUI显示错误

Question

    form = widgets.VBox()
first = widgets.Text(description="First:")
last = widgets.Text(description="Last:")

student = widgets.Checkbox(description="Student:", value=False)
school_info = widgets.VBox(visible=False, children=[
    widgets.Text(description="School:"),
    widgets.IntText(description="Grade:", min=0, max=12)
    ])

pet = widgets.Text(description="Pet:")
form.children = [first, last, student, school_info, pet]
display(form)

def on_student_toggle(name, value):
    if value:
        school_info.visible = True
    else:
        school_info.visible = False
student.on_trait_change(on_student_toggle, 'value')

我认为代码是正确的。但我的Jupyter笔记本刚出现错误，有什么问题？我该如何修理呢？