échafaudage de flottement.of () appelé avec un contexte qui ne contient pas d'échafaudage

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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