fix(dev_tools): prevent nil data error on app termination

[zanadoman]

Fixed strange error messages after terminating the application while using dev tools in browser.

Plugin configuration:

if vim.fn.exepath "flutter" ~= "" then
    vim.pack.add {
        "https://github.com/nvim-lua/plenary.nvim",
        "https://github.com/mfussenegger/nvim-dap",
        "https://github.com/zanadoman/flutter-tools.nvim"
    }
    require "flutter-tools".setup { dev_log = { enabled = false }, dev_tools = { autostart = true, auto_open_browser = true } }
end

Steps to reproduce

• Open lib/main.dart
• Launch app with FlutterRun
• Close app

vim.schedule callback: ...e/opt/flutter-tools.nvim/lua/flutter-tools/dev_tools.lua:131: attempt to get length of local 'data' (a nil value) stack traceback:
        ...e/opt/flutter-tools.nvim/lua/flutter-tools/dev_tools.lua:131: in function 'fn'
        [string "vim/_core/editor"]:273: in function <[string "vim/_core/editor"]:272>

I added the missing nil checks to bypass the errors, now 'Dev tools closed' message appears after closing the app without any issue. If you have a more elegant solution to solve this issue please modify my solution. :)

Thank you for your work!