Bug 动态切换主题后，Carousel的PageIndex未更新 #1744
Description
Describe the bug
Carousel控件UpdateItemsPosition()方法：动态切换主题时，Items.Count>0，但PageIndex未更新，闪退
Steps to reproduce the bug
动态切换主题后，当Items.Count=1，此时PageIndex并未更新，依然为-1，闪退，临时方案：额外的检查Items.Count大于0时：PageIndex = PageIndex < 0 ? 0 : PageIndex;
Expected behavior
No response
Screenshots
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NuGet package version
None
IDE
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Framework type
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Windows version
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Additional context
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