Fix crash in Linux platform channel example. (#155735)

When running this example it was crashing, so investigated a fixed a
couple of bugs.

Author: robert-ancell

examples/platform_channel/linux/my_application.cc

@@ -42,8 +42,7 @@ static void update_charging_state(MyApplication* self) {
 
   const gchar* charge_event = "discharging";
   for (guint i = 0; i < self->battery_devices->len; i++) {
-    UpDevice* device =
-        static_cast<UpDevice*>(g_ptr_array_index(self->battery_devices, i));
+    UpDevice* device = UP_DEVICE(g_ptr_array_index(self->battery_devices, i));
 
     guint state;
     g_object_get(device, "state", &state, nullptr);
@@ -97,8 +96,7 @@ static void up_device_removed_cb(MyApplication* self, UpDevice* device) {
 static FlMethodResponse* get_battery_level(MyApplication* self) {
   // Find the first available battery and use that.
   for (guint i = 0; i < self->battery_devices->len; i++) {
-    UpDevice* device =
-        static_cast<UpDevice*>(g_ptr_array_index(self->battery_devices, i));
+    UpDevice* device = UP_DEVICE(g_ptr_array_index(self->battery_devices, i));
 
     double percentage;
     g_object_get(device, "percentage", &percentage, nullptr);
@@ -225,8 +223,7 @@ static void my_application_activate(GApplication* application) {
   g_autoptr(GPtrArray) devices = up_client_get_devices(self->up_client);
 #endif
   for (guint i = 0; i < devices->len; i++) {
-    g_autoptr(UpDevice) device =
-        static_cast<UpDevice*>(g_ptr_array_index(devices, i));
+    UpDevice* device = UP_DEVICE(g_ptr_array_index(devices, i));
     up_device_added_cb(self, device);
   }
 
@@ -268,8 +265,7 @@ static void my_application_dispose(GObject* object) {
   MyApplication* self = MY_APPLICATION(object);
 
   for (guint i = 0; i < self->battery_devices->len; i++) {
-    UpDevice* device =
-        static_cast<UpDevice*>(g_ptr_array_index(self->battery_devices, i));
+    UpDevice* device = UP_DEVICE(g_ptr_array_index(self->battery_devices, i));
     g_signal_handlers_disconnect_matched(device, G_SIGNAL_MATCH_DATA, 0, 0,
                                          nullptr, nullptr, self);
   }
@@ -278,7 +274,7 @@ static void my_application_dispose(GObject* object) {
 
   g_clear_pointer(&self->dart_entrypoint_arguments, g_strfreev);
   g_clear_object(&self->up_client);
-  g_clear_object(&self->battery_devices);
+  g_clear_pointer(&self->battery_devices, g_ptr_array_unref);
   g_clear_object(&self->battery_channel);
   g_clear_object(&self->charging_channel);
   g_clear_pointer(&self->last_charge_event, g_free);

examples/platform_channel/test/platform_channel_test.dart

@@ -0,0 +1,15 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:platform_channel/main.dart' as platform_channel;
+
+void main() {
+  testWidgets('Platform channel smoke test', (WidgetTester tester) async {
+    platform_channel.main(); // builds the app and schedules a frame but doesn't trigger one
+    await tester.pump(); // triggers a frame
+
+    expect(find.textContaining('Battery level: '), findsOneWidget);
+  });
+}