Flutter Scaffold.Of () chamado com um contexto que não contém um andaime

This exception happens because you are using the context of the widget that
instantiated Scaffold.
Not the context of a child of Scaffold.
You can solve this by just using a different context :

Scaffold(
    appBar: AppBar(
        title: Text('SnackBar Playground'),
    ),
    body: Builder(
        builder: (context) => 
            Center(
            child: RaisedButton(
            color: Colors.pink,
            textColor: Colors.white,
            onPressed: () => _displaySnackBar(context),
            child: Text('Display SnackBar'),
            ),
        ),
    ),
);
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