下面是表格：通過為某些表單字段發送空值來努力驗證表單。姓名和電話是必填字段。當用戶鍵入并單擊提交時，應將所有值連接起來并保存在文件中。表格.pyfrom django import formsclass ReviewForm1(forms.Form):    PRECAUTIONS_CHOICES = (        ('no', 'No'),        ('yes', 'Yes')    )    UNIT1_CHOICES = (        ('Very Satisfied', 'Very Satisfied'),        ('Satisfied', 'Satisfied'),        ('neutral', 'neutral'),        ('Unsatisfied', 'Unsatisfied'),        ('Very Unsatisfied', 'Very Unsatisfied'),    )    name = forms.CharField(required=True, widget=forms.TextInput(attrs={'required': 'true'}))    phone = forms.CharField(required=False)    email = forms.EmailField(required=True, widget=forms.TextInput(attrs={'required': 'true'}))    precautions = forms.ChoiceField(required=True, choices= PRECAUTIONS_CHOICES, widget=forms.Select())    unit1_ambience = forms.ChoiceField(required=True, choices= UNIT1_CHOICES, widget=forms.Select())    review = forms.CharField(required=False, widget=forms.Textarea())    def clean(self):        name = self.cleaned_data.get('name')        phone = self.cleaned_data.get('phone')        review = self.cleaned_data.get('review')        email = self.cleaned_data.get('email')        precautions = self.cleaned_data.get('precautions')        unit1_ambience = self.cleaned_data.get('unit1_ambience')        expected_string = ''        options = [name,phone,review,email,precautions,unit1_ambience]        for option in options:            if not option:                continue            else:                expected_string = expected_string + option + '\n'        # concat all with review and then return string        return expected_string以下是單擊保存時出現的錯誤：賦值前引用的局部變量“review”我認為 form.is_valid() 返回 false，如何解決這個問題？