函数返回None或结束时没有返回语句

Question

@app.route('/createProduct', methods=['GET', 'POST'])
def createProduct():
    createProductForm = CreateProduct(request.form)
    if request.method == 'POST' and createProductForm.validate():
        itemsDict = {}
        db = shelve.open('items.db', 'c')
        try:
            itemsDict = db['Product']
            Product.Product.countID = db['CountID']
        except:
            print('Error in retrieving Items from items.db.')
        if 'file' not in request.files:
            flash('No file part')
            return redirect(request.url)
        file = request.files['files']
        if file.filename == '':
            flash('No selected file')
            return redirect(request.url)
        if file and allowed_file(file.filename):
            filename = secure_filename(str(createProductForm.Product.itemID.data + '.jpg'))
            file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))

            item = Product.Product(createProductForm.itemID.data, createProductForm.name.data,
                                   createProductForm.price.data, createProductForm.color.data,
                                   createProductForm.size.data, createProductForm.quantity.data,
                                   createProductForm.gender.data, createProductForm.description.data)

            itemsDict[item.get_itemID()] = item
            db['Product'] = itemsDict
            db['CountID'] = Product.Product.countID
            db.close()

            return redirect(url_for('retrieveProducts'))
        return render_template('createProduct.html', form=createProductForm, status='admin')

当我运行上面的代码时，会弹出这个窗口--&燃气轮机； “查看函数未返回有效响应。错误” TypeError:view函数未返回有效响应。函数返回None或结束时没有返回语句