glowl fetal cause NullReferenceException #848

Describe the bug
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To Reproduce
Steps to reproduce the behavior:

1. Go to HandyControlDemo
2. Click on 信息通知-->严重
3. Scroll down to '....'
4. See error NullReferenceException cause app fetal

Expected behavior
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Screenshots
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Environment (please complete the following information):

• .net: all version of netcore and nfx
• IDE [e.g. vs2019]
• Version [e.g. 1.0.0]

Additional context
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