codediff integration broken by codediff.nvim 2.67.9 SessionConfig changes: "attempt to index local 'ref'"

Problem

Every entry point that opens codediff from neogit crashes, e.g. pressing
dd on a commit in a log buffer:

vim.schedule callback: .../codediff.nvim/lua/codediff/ui/view/helpers.lua:54:
attempt to index local 'ref' (a nil value)
stack traceback:
        .../codediff.nvim/lua/codediff/ui/view/helpers.lua:54: in function 'prepare_buffer'
        .../codediff.nvim/lua/codediff/ui/view/side_by_side.lua:98: in function 'open_diff_panes'
        .../codediff.nvim/lua/codediff/ui/view/side_by_side.lua:336: in function 'create'
        .../neogit/lua/neogit/integrations/codediff.lua:86: in function 'open_explorer'
        .../neogit/lua/neogit/integrations/codediff.lua:110: in function <...:109>

Environment

• neogit: master (5adc81b2)
• codediff.nvim: >= 2.67.9 (breaking change introduced in b263f54,
  "refactor(session): fold the panel object into the panel descriptor")

Reproduce

1. diff_viewer = "codediff" (or auto-detected with codediff.nvim installed)
2. Open any log buffer (NeogitLogCurrent / Neogit log)
3. Press dd on a commit

Start in lua/neogit/integrations/codediff.lua, especially open_explorer around the reported line, and compare the arguments it passes with codediff.nvim's SessionConfig changes from b263f54. Reproduce with codediff enabled by opening a log buffer and pressing dd on a commit; done means each codediff entry point opens without the nil ref error.