ranking的demo中初始化未赋值

### 描述Bug

```
//此接口创建对象后并未对私有变量openDataContext赋值。
private void InitOpenDataContext()
{
    if (openDataContext == null)
    {
        WXOpenDataContext openDataContext = WX.GetOpenDataContext(new OpenDataContextOption
        {
            sharedCanvasMode = selectedCanvasType
        });
    }
}

```

### 运行环境

```Text
windows
```
