Doing this results in a Error running timer: (wrong-type-argument commandp lsp-treemacs-list-errors-quick-fix)

The reason seems to be that in lsp-treemacs--error-list-diags the actions defined name lsp-treemacs-list-errors-quick-fix as action, while the action definition below this is lsp-treemacs-quick-fix, and lsp-treemacs-list-errors-quick-fix does not exist. Changing the action name in the actions definition resolves the problem