How to show a progress Dialog before data loading in flutter?

Make sure that _category is null, maybe you're assigning a value to it before loading the data.

Use a FutureBuilder to control the rendering during the load time;

  final categories = Webservice().load(Category.allCategory);

  Widget build(BuildContext context) {
    return FutureBuilder(
      future: categories,
      builder: (ctx, snapshot) {
        var value = (snapshot.connectionState == ConnectionState.done) ? '${_category.catToDo}' : '0';

        return Text(
          value,
          style: TextStyle(
            fontSize: 30,
            fontWeight: FontWeight.bold
          ),
        );
      }
    );
  }

Or if you want to display a loading animation :

  final categories = Webservice().load(Category.allCategory);

  Widget build(BuildContext context) {
    return FutureBuilder(
      future: categories,
      builder: (ctx, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          return Text(
            '${_category.catToDo}',
            style: TextStyle(
              fontSize: 30,
              fontWeight: FontWeight.bold
            ),
          );
        }
        else {
          return CircularProgressIndicator();
        }
      }
    );
  }

You can check this package to show a loading spin with different styles.

After that you need to use Future Builder widget

Here is an example of how to use it with the spinkit

FutureBuilder(
        future: myAwesomeFutureMethod(), // you should put here your method that call your web service
        builder:

            (BuildContext context, AsyncSnapshot<List<BillResponse>> snapshot) { 
            /// The snapshot data type have to be same of the result of your web service method
          if (snapshot.hasData) {
            /// When the result of the future call respond and has data show that data
            return SingleChildScrollView(
              scrollDirection: Axis.horizontal,
              child: bodyData(snapshot.data),
            );
          }
          /// While is no data show this
          return Center(
            child: SpinKitDualRing(
              color: Colors.blue,
            ),
          );
        },
      ),

Hope this could help. Cheers.