fix(panel): fixed an exception that occurred when the title attribute was empty

Author: 2534290808

example/lib/panel_example.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_weui/flutter_weui.dart';
+
+class MediaBoxExample extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text("Cell"),
+      ),
+      body: Cells(
+        children: <Widget>[
+          MediaBox(
+            title: "Title",
+            desc: "desc",
+          ),
+          MediaBox(
+            thumb: Container(
+              width: 24,
+              height: 24,
+              color: Colors.red,
+              child: Text("A"),
+            ),
+            title: "Title",
+            desc: "desc",
+          ),
+          MediaBox(
+            title: "Title",
+            desc: "desc",
+            info: Info(children: <Widget>[
+              Meta(text: "文字来源",),
+              Meta(text: "文本来源",),
+              MetaExtra(text: "其他信息",)
+            ],),
+          )
+        ],
+      ),
+    );
+  }
+}

lib/panel.dart

@@ -19,7 +19,7 @@ class Panel extends StatelessWidget {
           Offstage(
             child: Container(
               padding: const EdgeInsets.only(top: 14, right: 15, bottom: 10, left: 15),
-              child: Text(title, style: titleStyle),
+              child: Text(title??"", style: titleStyle),
             ),
             offstage: title == null,
           ),