You should select only Widget! part like (without trailing comma)

// Source - https://stackoverflow.com/q/56558409
// Posted by Nicholas Muir, modified by community. See post 'Timeline' for change history
// Retrieved 2025-11-20, License - CC BY-SA 4.0

Expanded(
  child: Container(
    child: Padding(
      padding: const EdgeInsets.only(left: 8.0, right: 8.0, top: 8.0, bottom: 4.0),
      child: TextField(
        decoration: InputDecoration(
          border: OutlineInputBorder(
            borderRadius: const BorderRadius.all(
              const Radius.circular(10.0),
            ),
          ),
          filled: true,
          hintStyle: TextStyle(color: Colors.grey[800]),
          hintText: "Supervisor",
          fillColor: Colors.white70,
        ),
      ),
    ),
  ),
) //, (without this comma and it will work)