通过PackageInfo获取ActivityInfo都是null

// 获取PackageManager
PackageManager pm = getPackageManager();

// 获取PackageInfo。PackageManager.GET_UNINSTALLED_PACKAGES与PackageManager.GET_ACTIVITIES好像不能同时使用，会报错。
List<PackageInfo> pkgs=pm.getInstalledPackages(PackageManager.GET_UNINSTALLED_PACKAGES);

// 下面代码获取不到对应的ActivityInfo,actInfo总是null,
//ActivityInfo[] actInfo = pkgs.get(0).activities;

//这样就能获取ActivityInfo了，之后可以获得Activity的name
ActivityInfo[] actInfo = getPackageManager().getPackageInfo(pkgs.get(0).packageName, PackageManager.GET_ACTIVITIES).activities;