null exception when calling getKey

[EbenZhang]

Describe the bug
null exception when calling getKey

E/flutter ( 4581): [ERROR:flutter/shell/common/shell.cc(184)] Dart Error: Unhandled exception:
E/flutter ( 4581): NoSuchMethodError: The method 'getStringList' was called on null.
E/flutter ( 4581): Receiver: null

To Reproduce
Steps to reproduce the behavior:

var cache = new CsCache();
cache.getKey("key");

Expected behavior

Able to call getKey without exception

Desktop (please complete the following information):

• OS: Mint19
• Version of this package: ^1.0.0

Smartphone (please complete the following information):

• Device: Android emulator Pixel XL API 28, Android 9.0 (with Google APIs) x86

Additional context

I guess the getKey was called before the async _setSharedPref finished.

factory CsCache() {
......
     _singltonInstance._setSharedPref();
......
}
void _setSharedPref() async {
    _sharedPreferences = await SharedPreferences.getInstance();
}

BTW, using Flutter 1.0