Doing some testing on the "Optimistic State" I noticed the use of Dismissable was incorrect.

If the delete request failed and the item cannot be removed, the removed item then gets stays in the item list.

This causes the Dismissable widget to throw an exception, because it expects that the item with a unique key has been removed. To fix that, I switched from onDismissed to confirmDismiss, and use the Command status result to know if the dismiss action was successful or not.

I have created a widget test that reproduces the issue.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

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