title: Problem using Slidable with onPressed. (flutter_slidable: ^1.2.0)
body: <p>I am having trouble with <code>flutter_slidable: ^1.2.0</code>.<br />
  Looking at the code below <br /></p>
  <pre><code>import 'package:flutter/material.dart';
  import 'package:flutter/widgets.dart';
  import 'package:flutter_slidable/flutter_slidable.dart';
  
  class SlidableWidget extends StatelessWidget {
    final Widget child;
    final void Function(BuildContext context) onDismissed;
  
    const SlidableWidget({required this.child, required this.onDismissed, Key? key}) : super(key: key);
  
    @override
    Widget build(BuildContext context) =&gt; Slidable(
          child: child,
          endActionPane: const ActionPane(motion: ScrollMotion(), children: [
            SlidableAction(
              flex: 1,
              onPressed: this.onDismissed(context),
              backgroundColor: Colors.red,
              foregroundColor: Colors.white,
              icon: Icons.delete,
              label: 'Delete',
            ),
          ],
          extentRatio: 0.2,),
        );
  }
  </code></pre>
  <p>I want <code>onDismissed</code> function to be passed as a parameter but on the line <code>onPressed: this.onDismissed(context)</code>, it isn't right. It says <code>Invalid const value</code>. But I don't know what is wrong and how to solve it.  Can anyone explain me?
  <br />
  <br />
  Thank you so much.<br />
  <br />
  Sura</p>