Describe the bug
调用MessageBox的Ask,Error,Info几个API均会导致崩溃现象

To Reproduce
Steps to reproduce the behavior:

1. 编译库自带Demo,选择控件->窗口->打开消息窗口
   2.点击"打开消息窗口"按钮，会崩溃，代码定位到window.cs 392行 _button为null
2. See error

Expected behavior
正确现象应为顺利弹出窗口显示
Screenshots
Environment (please complete the following information):

• .net: [e.g. 4.8]
• IDE [e.g. vs2019]
• Version [3.3.9]

Additional context
Add any other context about the problem here.