The method 'getProducts' was called on null - flutter

Question

i am trying to retrieve data using rest api from woocommerce website using flutter
this is the api for retrieve json data
Future<List<Product>> getProducts(String tagId) async {
List<Product> data = new List<Product>();
try {
String url = Config.url +
Config.productsURL +
"?consumer_key=${Config.key}&consumer_secret=${Config.secret}&tag=$tagId";
var response = await Dio().get(url,
options: new Options(
headers: {HttpHeaders.contentTypeHeader: "application/json"}));
if (response.statusCode == 200) {
data = (response.data as List).map((i) => Product.fromJson(i),).toList();
}
} on DioError catch (e) {
print(e.response);
}
return data;
}
this is the widget to handle the data to the mobile app
class WidgetHomeProducts extends StatefulWidget {
WidgetHomeProducts({Key key, this.labelName, this.tagId}) : super(key : key);
String labelName;
String tagId;
#override
_WidgetHomeProductsState createState() => _WidgetHomeProductsState();
}
class _WidgetHomeProductsState extends State<WidgetHomeProducts> {
APIServices apiServices;
#override
void initState() {
apiServices = new APIServices();
super.initState();
}
#override
Widget build(BuildContext context) {
return Container(
color: const Color(0xffF4F7FA),
child: Column(
children: <Widget>[
Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Padding(
padding: EdgeInsets.only(left: 16, top: 4),
child: Text(
this.widget.labelName,
style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
),
),
Padding(
padding: EdgeInsets.only(left: 16, top: 4),
child: FlatButton(
onPressed: () {},
child: Text(
'View All',
style: TextStyle(color: Colors.blueAccent),
),
),
),
],
),
_productList(),
],
),
);
}
Widget _productList(){
return new FutureBuilder(
future: apiServices.getProducts(this.widget.tagId),
builder: (BuildContext context, AsyncSnapshot<List<Product>> model){
if(model.hasData){
return _buildList(model.data);
}if(model.hasError){
print("error");
}
return Center(child: CircularProgressIndicator(),);
});
}
i got The method error message that says
'getProducts' was called on null.
Receiver: null
Tried calling: getProducts("971")
can anyone help me to fix this?