Non-UTF8 characters appear to break the semantic tokens feature

[pagedMov]

Describe the bug
Semantic tokens appear to be broken by files that contain non UTF-8 characters.

Logs (Required)

Error executing vim.schedule lua callback: ....10.2/share/nvim/runtime/lua/vim/lsp/semantic_tokens.lua:304: ....10.2/share/nvim/runtime/lua/vim/lsp/semantic_tokens.lua:149
: index out of range
stack traceback:
[builtin#36]: at 0x7f16a7547e00
....10.2/share/nvim/runtime/lua/vim/lsp/semantic_tokens.lua:304: in function 'handler'
...wrapped-0.10.2/share/nvim/runtime/lua/vim/lsp/client.lua:687: in function ''
vim/_editor.lua: in function <vim/_editor.lua:0>

Configuration

I use nixvim to configure nixd. These are my settings:

nixd = {
  enable = true;
  settings = {
    nixd = {
      nixpkgs.expr = "import <nixpkgs> { }";
      formatting.command = "nixfmt";
    };
    options = {
      nixos.expr = "(builtins.getFlake \"github:pagedMov/nixos-config\").nixosConfigurations.xenon.options";
      home.expr = "(builtins.getFlake \"github:pagedMov/nixos-config\").homeConfigurations.xenonHome.options";
    };
  };
};

To Reproduce
Steps to reproduce the behavior:

1. Open a file containing non UTF-8 characters (Here is the file I've been using to test fixes/workarounds, it triggers this bug every time)
2. Attempt to do anything that makes calls to the LSP.

[inclyc]

Duplicate of #604.