[QA] 必须消除所有警告后提交代码

### 问题

```bash
flutter analyze
```

->

```
...
warning - The operand can't be 'null', so the condition is always 'true' - lib\pages\calendar_page.dart:69:23 - unnecessary_null_comparison
...
warning - The declaration '_buildErrorView' isn't referenced - lib\pages\calendar_page.dart:788:10 - unused_element
...
```

### 建议

建议所有开发者提交代码前解决所有编译警告。
