use FlutterBoostFragment made Navigator.push() is not work.

## Steps to Reproduce
**A small application to reproduce the bug(最小化可复现的demo)**

1. A Activity contain a ViewPager ，ViewPager has two item,one is Fragment ,another is FlutterBoostFragment。
2. The Fragment adapter is
```
public class BaseFragmentPagerAdapter extends FragmentPagerAdapter {
protected List mList;

public BaseFragmentPagerAdapter(FragmentManager fm, List list) {
// 这里的第二个参数 不可以取消 FlutterBoostFragment 会崩溃
super(fm, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
}
```
3. when I open this Activity From FlutterPage ，then back to FlutterPage ，then try to Navigator.push() jump page,but it is notwork.
4. when I set FlutterBoostFragment is First item, Navigator.push() worked.

**Flutter Boost Version**
**Target Platform:**
**Target OS version/browser:**
**Devices:**

## Logs
add your crash log or something else.

```
```

Start with the FlutterBoostFragment and the Android Activity/ViewPager setup described in the reproduction, then trace the FlutterPage return path and Navigator.push() behavior. Compare the working case where FlutterBoostFragment is the first item with the failing pager order; done means navigation works after returning to FlutterPage without a crash.

Written by the indexing model from the issue text.