Merge pull request #380 from hengkx/master

SharedPreferences 两个地方同时调用导致_spf为null而取不到值

lib/utils/shared_preferences.dart

@@ -24,8 +24,9 @@ class SpUtil {
   static Future<SpUtil> getInstance() async  {
     if (_instance == null) {
       _instance = new SpUtil._();
+    }
+    if (_spf == null) {
       await _instance._init();
-
     }
     return _instance;
   }
@@ -117,4 +118,4 @@ class SpUtil {
     if (_beforeCheck()) return null;
     return _spf.clear();
   }
-}
+}