Update simple_builder.dart

@@ -5,7 +5,7 @@ import 'list_notifier.dart';
 
 
 typedef ValueBuilderUpdateCallback<T> = void Function(T snapshot);
 typedef ValueBuilderUpdateCallback<T> = void Function(T snapshot);
 typedef ValueBuilderBuilder<T> = Widget Function(
 typedef ValueBuilderBuilder<T> = Widget Function(
-    T snapshot, ValueBuilderUpdateCallback<T> updater);
+    T? snapshot, ValueBuilderUpdateCallback<T> updater);
 
 
 /// Manages a local state like ObxValue, but uses a callback instead of
 /// Manages a local state like ObxValue, but uses a callback instead of
 /// a Rx value.
 /// a Rx value.