3.0原生跳flutter后，返回再跳一次会崩溃

#### 日志：
```
java.lang.RuntimeException: Unable to resume activity {com.baitu.poppo/com.idlefish.flutterboost.containers.FlutterBoostActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'android.app.Activity io.flutter.embedding.android.FlutterActivityAndFragmentDelegate$Host.getActivity()' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4323)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4355)
at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:52)
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:176)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2110)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7697)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
```
不知道是什么原因引起的，流程是按照文档的步骤接入的 @0xZOne

Start with the integration flow described in the documentation and reproduce the sequence of jumping from the native app to Flutter, returning, then jumping again. Read the FlutterBoostActivity lifecycle and the FlutterActivityAndFragmentDelegate stack-trace locations; done means the second navigation resumes without the reported NullPointerException.

Written by the indexing model from the issue text.