Problem:
Exiting the insert mode with ctrl-c does not trigger InsertLeave
autocmd. This may lead to nil error in treesitter foldexpr.
Solution:
Check nil. Folds still can be stale after exiting the insert mode with
ctrl-c, but it will be eventually updated correctly.
An alternative solution would be to ensure that exiting the insert mode
always triggers do_foldupdate. This can be done either by "fixing"
ctrl-c or with on_key callback that checks ctrl-c (nvim-cmp does this).
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