Mostrar dados da api

API Encontrei -Get Categorias

Future<Map<String, dynamic>> _fetchCategoriesPreview() async {
    http.Response response =
        await http.get(Uri.encodeFull(UtilsURL.CATEGORIES_PREVIEW), headers: {
      "Authorization": 'Token 9dcc473d8e65ccdd6440e03f0622642b11acbbbd',
      "Accept": "application/json"
    });
    if (response.statusCode == 200) {
//      return json.decode(response.body);
      return json.decode(response.body[3]);
    } else {
      throw Exception('Erro: ' + response.statusCode.toString());
    }
  }

  Widget _sliverGridCategory() {
    return FutureBuilder<Map<String, dynamic>>(
      future: _fetchCategoriesPreview(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return SliverList(
              delegate: SliverChildListDelegate([
            Center(
              child: CircularProgressIndicator(),
            )
          ]));
        } else {
          return SliverGrid(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                crossAxisCount: 2,
                mainAxisSpacing: 4.0,
                childAspectRatio: 1.0,
                crossAxisSpacing: 4.0),
            delegate:
                SliverChildBuilderDelegate((BuildContext context, int index) {
              return InkWell(
                onTap: () {
                  //Navigator.of(context).push(MaterialPageRoute(
                  //   builder: (context) => CategoriesActivity(categoryData, snapshot)));
                },
                child: Card(
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(20.0)),
                    child: ClipRRect(
                      borderRadius: BorderRadius.circular(20.0),
                      child: Stack(
                        alignment: Alignment.bottomCenter,
                        children: <Widget>[
                          AspectRatio(
                              aspectRatio: 1.0,
                              child: Image.network(
                                snapshot.data['imagem'],
                                fit: BoxFit.cover,
                              )),
                          Container(
                            height: 25.0,
                            width: MediaQuery.of(context).size.width,
                            decoration: BoxDecoration(
                                color: Color.fromARGB(180, 0, 0, 0)),
                            child: Padding(
                              padding: EdgeInsets.symmetric(vertical: 4.0),
                              child: Text(
                                snapshot.data['results']['titulo'],
                                textAlign: TextAlign.center,
                                style: TextStyle(
                                    fontWeight: FontWeight.w700,
                                    color: Colors.white),
                              ),
                            ),
                          ),
                        ],
                      ),
                    )),
              );
            }, childCount: snapshot.data.length),
          );
        }
      },
    );
  }