Fatal error: Unsupported operand types in webform_client_form_submit()

Create a 2 page form. Set it to save drafts when moving between pages. Go to the second page. Delete the draft. Go to the previous page. You will get a whitescreen, "Fatal error: Unsupported operand types" because webform_client_form_submit() is trying to use the + operator between the submitted data array and non-existent $submission->data.