如果是在Activity中，获取FragmentManager是通过getSupportFragmentmanager方法来获取
如果是在Fragment中切换子Fragment是通过getChildFragmentManager方法来获取 否则这里加载会出问题
在Fragment中切换子级Fragment示例代码如下：

//step1:获取fm
 FragmentManager fragmentManager = getChildFragmentManager();
//step2 开启事务
 FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
//切换Fragment   隐藏当前Fragment
fragmentTransaction.hide(nowFragment);
//判断fragment是否已经被加载
 if(!toFragment.isAdded()){
            fragmentTransaction.add(R.id.container,toFragment).commit();
 }else{
            fragmentTransaction.show(toFragment).commit();
 }
 if(nowFragment!=toFragment){
         nowFragment=toFragment ;
}