Develop nullsafety

[talisk]

Describe what problem this pull request solves and how to solve it. If there is an issue associated with it, please attach a link.

请描述这个 pull request 解决了什么问题，以及解决方式。如果有与之关联的 issue，请附链接。

Pull Request Checklist

• I have read the UME contribution document and understand how to contribute, commit the code according to the rules. 我已阅读过 UME 贡献文档，并了解如何进行贡献，按照规则提交了代码
• I have added the necessary comments in code to ensure that other contributors can understand the reason for the change. 我在修改中已经添加了必要的注释，以确保便于其他贡献者理解修改原因
• The code has been formatted by dartfmt before push. 代码在提交前已经经过 dartfmt 进行了格式化
• Change does not involve the adjustment of test cases. Or all existing and new tests are passing. 改动不涉及测试用例调整，或 example 工程与单元测试已经完全跑通

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