Check for empty list

lib/widgets/drop_down.dart

@@ -10,7 +10,7 @@ class DropDown extends StatelessWidget {
   Widget build(BuildContext context) {
     var inputModel = context.watch<InputProvider>();
     return DropdownButtonFormField<String>(
-      value: _items[0],
+      value: _items.isEmpty ? null : _items[0],
       icon: const Icon(Icons.arrow_downward),
       elevation: 16,
       style: const TextStyle(color: Colors.deepPurple),