解决swift工程引入问题，解决don't support null safety问题

Flutter/ios/super_player.podspec

@@ -18,6 +18,7 @@ player plugin.
   s.dependency 'Flutter'
   s.dependency 'SuperPlayer'
   s.platform = :ios, '8.0'
+  s.static_framework = true
 
   # Flutter.framework does not contain a i386 slice.
   s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386' }

Flutter/lib/Core/superplayer_plugin.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class SuperPlayerPlugin {

Flutter/lib/Core/superplayer_widget.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 typedef void SuperPlatformViewCreatedCallback(SuperPlayerPlatformViewController controller);

Flutter/lib/Core/superplayerview_config.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class SuperPlayerViewConfig {

Flutter/lib/Core/superplayerview_controller.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class SuperPlayerPlatformViewController {

Flutter/lib/Core/superplayerview_model.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class SuperPlayerViewModel {

Flutter/lib/Core/txliveplayer_controller.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class TXLivePlayerController extends ChangeNotifier implements ValueListenable<TXPlayerValue>, TXPlayerController {

Flutter/lib/Core/txplayer_controller.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 abstract class TXPlayerController {

Flutter/lib/Core/txplayer_define.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class TXPlayerValue{

Flutter/lib/Core/txplayer_widget.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class TXPlayerVideo extends StatefulWidget {

Flutter/lib/Core/txvodplayer_controller.dart

+// @dart = 2.7
 part of SuperPlayer;
 
 class TXVodPlayerController extends ChangeNotifier implements ValueListenable<TXPlayerValue>, TXPlayerController {

Flutter/lib/super_player.dart

+// @dart = 2.7
 library SuperPlayer;
 
 import 'dart:async';