fix: StyledBehindWindowBlur crash in render thread

[18202781743]

as title.

pms: TASK-368399

Summary by Sourcery

Bug Fixes:

• Fixes a crash in the render thread that could occur when the blur behind item is destroyed by a Loader by checking if the window is valid before rendering.

[sourcery-ai]

Hey @18202781743 - I've reviewed your changes - here's some feedback:

Overall Comments:

• It might be good to add a comment explaining why m_item->window() might be null.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

---

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[deepin-bot]

TAG Bot

New tag: 5.7.11
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #470

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[deepin-ci-robot]

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