修复bug:

1. android 4.4 首页底栏不展现
2. 重新登录后ActionSheet失效

src/page/HomePage.js

@@ -119,7 +119,10 @@ export default class HomePage extends Component {
                   }
                 },
                 bottomTabs: {
-                    visible: true
+                    visible: true,
+
+                    drawBehind: false,
+                    animate: false
                 }
             });
             

src/page/NotificationPage.js

@@ -147,7 +147,9 @@ export default class NotificationPage extends Component {
     }
 
     refresh() {
-        this.list.refresh(false);
+        if(this.list) {
+            this.list.refresh(false);
+        }
     }
 
     _onDeletePress(msg) {

src/page/SettingPage.js

@@ -114,6 +114,14 @@ export default class SettingPage extends Component {
         Alert.alert('提示','确认退出登录?',[
             {text: '退出', style: 'destructive', onPress: () => {
                 Api.logout();
+
+                /*
+                *  clear ActionSheet instance
+                */
+                if(global.__action_sheet) {
+                    global.__action_sheet = null;
+                }
+
                 Navigation.setRoot({
                     root: {
                         stack: {