'new_form_id() -> The id for the next member of the formset. Increment hidden value.
The ManagementForm needs to keep track of a monotonically increasing id, so that
new member forms don\'t reuse ids of deleted forms.'

def new_form_id(self):

    res = self.form_counts()
    data2 = self.data.copy()
    data2[self.add_prefix('next_form_id')] = str((res + 1))
    self.data = data2
    return res
