从TextEditingController获取文本的唯一方法是在Controller的末尾插入get方法.text，记住这一点，您的代码应该如下所示：

class AddProduct extends StatelessWidget {

final _formKey = GlobalKey<FormState>();
final TextEditingController _nameController = TextEditingController();
final TextEditingController _descriptionController = TextEditingController();
final TextEditingController _priceController = TextEditingController();
final TextEditingController _imageUrlController = TextEditingController();

Future saveProduct() async {
  final response = await http.post(Uri.parse("http://127.0.0.1:8000/api/products"), 
  body: {
    "name": _nameController.text,
    "description": _descriptionController.text,
    "price": _priceController.text,
    "image_url": _imageUrlController.text
    });
}

请注意，检查输入字段是否不为空是一个好主意，除非出于某种原因，您不关心用户是否发送了一些空数据。