Listen to oncharacteristicvaluechanged as well.

It is ambiguous what event needs to be handled.

(Google chrome samples)[https://googlechrome.github.io/samples/web-bluetooth/notifications.html] uses `characteristicvaluechanged`.
While the (Web api draft)[https://webbluetoothcg.github.io/web-bluetooth/#dom-characteristiceventhandlers-oncharacteristicvaluechanged] uses `oncharacteristicvaluechanged`.

I haven't got either one to work with the devices on my range though.

Fixed bug in example where it would not update the `notify` button once it has started.

Fixed bug where example couldn't handle a service or characteristic that isn't in the default list.

example/lib/widgets/characteristic_actions.dart

@@ -107,6 +107,9 @@ class ActionsSate extends State<ActionsWidget> {
     return () async {
       try {
         await method.call();
+        setState(() {
+          // Make sure `isNotifying` is updated.
+        });
       } catch (e, s) {
         debugPrint("$e\n$s");
         if (mounted) {

example/lib/widgets/characteristic_widget.dart

@@ -9,7 +9,8 @@ class CharacteristicWidget extends StatefulWidget {
   CharacteristicWidget({required this.characteristic, super.key}) {
     characteristicName = BluetoothDefaultCharacteristicUUIDS.values
         .cast<BluetoothDefaultCharacteristicUUIDS?>()
-        .firstWhere((final element) => element?.uuid == characteristic.uuid)
+        .firstWhere((final element) => element?.uuid == characteristic.uuid,
+            orElse: () => null)
         ?.name;
   }
 

example/lib/widgets/service_widget.dart

@@ -13,7 +13,8 @@ class ServiceWidget extends StatefulWidget {
   ServiceWidget({required this.service, super.key}) {
     serviceName = BluetoothDefaultServiceUUIDS.values
         .cast<BluetoothDefaultServiceUUIDS?>()
-        .firstWhere((final element) => element?.uuid == service.uuid)
+        .firstWhere((final element) => element?.uuid == service.uuid,
+            orElse: () => null)
         ?.name;
   }
 

lib/src/bluetooth_characteristic.dart

@@ -32,6 +32,13 @@ class BluetoothCharacteristic {
         _value.add(data);
       }
     });
+    _characteristic.addEventListener("oncharacteristicvaluechanged",
+        (final event) {
+      final data = _characteristic.value;
+      if (data != null) {
+        _value.add(data);
+      }
+    });
   }
 
   final WebBluetoothRemoteGATTCharacteristic _characteristic;