Reddit: https://www.reddit.com/r/vim/comments/99utc7/ccls_languageclientneovim_custom_cross_references/
I finally identified and fixed the null pointer dereference issue of Arch Linux aur/ccls-git, damn -fno-rtti + std::get_deleter https://github.com/MaskRay/ccls/issues/30#issuecomment-415594762