fix: 修复导致 swiper item 重叠的问题（#5354）

[tofrankie]

关联 issue：#5354 (comment)

[Copilot]

Pull request overview

This PR fixes a critical null reference bug in App platform components that use the useRebuild callback. The issue (#5354) originally manifested in the swiper component where slideFrameRef.value was null when accessed during rebuild, causing errors and swiper-item overlap. This PR extends the fix pattern to other components that have the same vulnerability.

Changes:

• Add null safety checks for ref values in useRebuild callbacks to prevent null reference errors
• Ensures components can handle early callback invocation before DOM elements are attached to refs
• Maintains consistency with the fix pattern already established in swiper and picker-view components

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/uni-components/src/vue/movable-area/index.tsx	Adds null check for rootRef.value before accessing its children property in the useRebuild callback
packages/uni-app-plus/src/view/components/cover-view/index.tsx	Adds null check for textRef.value before accessing its childNodes in the useRebuild callback

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