Server error when grading nonexistent flow

Same if accessing the grade book. This may happen if completing a flow in preview and then trying to grade it, where grading always uses the currently public/active branch.

Start by reproducing the server error when grading a nonexistent flow, including a flow completed in preview while another branch is public/active. Compare the grading path with the grade-book path and trace how each resolves the flow branch. Done means both paths handle a missing or non-public flow without a server error and retain the expected behavior for active flows.

Written by the indexing model from the issue text.