fix crash when killing app on iOS

Author: LinXunFeng

darwin/Classes/FvpPlugin.mm

@@ -135,6 +135,7 @@ + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
 #endif
     FlutterMethodChannel* channel = [FlutterMethodChannel methodChannelWithName:@"fvp" binaryMessenger:messenger];
     FvpPlugin* instance = [[FvpPlugin alloc] initWithRegistrar:registrar];
+    [registrar addApplicationDelegate:instance];
     [registrar publish:instance];
     [registrar addMethodCallDelegate:instance channel:channel];
     SetGlobalOption("MDK_KEY", "C03BFF5306AB39058A767105F82697F42A00FE970FB0E641D306DEFF3F220547E5E5377A3C504DC30D547890E71059BC023A4DD91A95474D1F33CA4C26C81B0FC73B00ACF954C6FA75898EFA07D9680B6A00FDF179C0A15381101D01124498AF55B069BD4B0156D5CF5A56DEDE782E5F3930AD47C8F40BFBA379231142E31B0F");
@@ -183,4 +184,8 @@ - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
 - (void)detachFromEngineForRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
   players.clear();
 }
+
+- (void)applicationWillTerminate:(UIApplication *)application {
+  players.clear();
+}
 @end