Crashed when back pressed

1. I pressed `Push React Native`. It shown nothing. then I chose `Present React Native`. It showed up as screenshot below.

Then I pressed back. It crashed with log

``` Process: com.airbnb.android.react.navigation.example, PID: 4901
java.lang.IllegalStateException: Trying to set a replacement startPostponedEnterTransition on ReactNativeFragment{9095d17 #21 id=0x7f0e007e}
at android.support.v4.app.Fragment.setOnStartEnterTransitionListener(Fragment.java:2466)
at android.support.v4.app.BackStackRecord.setOnStartPostponedListener(BackStackRecord.java:901)
at android.support.v4.app.FragmentManagerImpl.postponePostponableTransactions(FragmentManager.java:2229)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2152)
at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2104)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2008)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:710)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
```

Running on android N

Start by reproducing the Android N flow: choose Push React Native, choose Present React Native, then press back. Use the reported IllegalStateException and ReactNativeFragment transition path as the starting point; done means the back action no longer crashes and the navigation flow remains usable.

Written by the indexing model from the issue text.