ValidationError in form wizard with formsets

[gchp]

This issue was originally submitted on Trac (#20116). See the previous link for the full conversation, and a link to an initial fix.

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I encountered an issue with the form wizard when the first step is a formset and the user resends a later step after completing the wizard.

Scenario:

• Complete the wizard successfully
• Hit the browser back button (page usually comes from cache then)
• Send the last form again

Because the wizard's storage gets reseted after completion, the wizard will start with the first step. If the first step is a formset, its ManagementForm raises a ValidationError because the data is passed but doesn't contain the expected values.