Fix compile error (#9449)

examples/platform_channel/ios/Runner/AppDelegate.m

diff --git a/examples/platform_channel/ios/Runner/AppDelegate.m b/examples/platform_channel/ios/Runner/AppDelegate.m
index f4ee7da..d8fd0f5 100644
--- a/examples/platform_channel/ios/Runner/AppDelegate.m
+++ b/examples/platform_channel/ios/Runner/AppDelegate.m
@@ -69,7 +69,7 @@
 }
 
 - (void)sendBatteryStateEvent {
-  if (!_eventReceiver) return;
+  if (!_eventSink) return;
   UIDeviceBatteryState state = [[UIDevice currentDevice] batteryState];
   switch (state) {
     case UIDeviceBatteryStateFull: