Android端升级空安全后，Flutter页面 首次启动会闪下黑屏

无论是Activity，还是fragment界面首次都会闪一下黑屏

升级 flutter_boost 从 v3.0-preview.18 到 v3.0-null-safety-preview.18支持空安全版本后
Android端出现此问题
IOS端没有问题

1.跳转Activity:
`public void pushFlutterRoute(FlutterBoostRouteOptions options) {
Intent intent = new CommonFlutterActivity.CachedEngineIntentBuilder(CommonFlutterActivity.class)
.backgroundMode(FlutterActivityLaunchConfigs.BackgroundMode.opaque)
.destroyEngineWithActivity(false)
.uniqueId(options.uniqueId())
.url(options.pageName())
.urlParams(options.arguments())
.build(FlutterBoost.instance().currentActivity());
FlutterBoost.instance().currentActivity().startActivityForResult(intent, options.requestCode());
}`

2.fragment添加
`val mineFlutterFragment = FlutterBoostFragment.CachedEngineFragmentBuilder(FlutterBoostFragment::class.java)
.url("mine_flutterpage")
.build()

mFragments.append(R.id.minePage, mineFlutterFragment)`

暂无报错信息

Start by reproducing the first-launch black flash on Android using the pushFlutterRoute Activity path and the FlutterBoostFragment setup shown in the issue after upgrading from v3.0-preview.18 to v3.0-null-safety-preview.18. Compare the Activity and Fragment startup behavior with iOS, then verify that both Android entry points launch without the black screen and without introducing errors.

Written by the indexing model from the issue text.