fix: walk the parent chain safely in CentralDocPage

[add-uos]

问题

CentralDocPage 的 isFullScreen()/openFullScreen()/quitFullScreen()/onSheetCountChanged()（reader/uiframe/CentralDocPage.cpp :897/:908/:928/:976）均使用 parentWidget()->parentWidget()->parentWidget() 三级父链。parentWidget() 非虚且内部解引用 d 指针，任一级为空（嵌入场景/测试环境父链不足三级）即对空指针调用崩溃——末尾的 if (nullptr == mainWindow) 只能拦截第三级为空的情况。gdb 实测：setSidebarVisible（issue 164325 修复路径）触发 SIGSEGV at QWidget::parentWidget(this=0x0)。同文件 :976 的调用点之后用了 mainWindow && ... 判空，说明防护模式已知但未一致应用。

修复

新增文件内静态辅助 parentMainWindow(CentralDocPage *)：逐级向上取父并随时判空，最后再做 dynamic_cast；4 处调用点统一替换。

背景

该问题由 AI 辅助单测（PMS 缺陷热点回归，BUG164325 用例运行时 gdb 定位）发现。

Summary by Sourcery

Safely traverse CentralDocPage’s parent hierarchy before accessing the parent MainWindow.

Bug Fixes:

• Prevent crashes when CentralDocPage is embedded in a hierarchy with fewer than three parent widgets by safely resolving its parent MainWindow.
• Apply consistent null-safe parent-window handling to fullscreen and sheet-count updates.

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[deepin-ci-robot]

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

CentralDocPage now resolves its owning MainWindow through a null-safe helper, preventing crashes when embedded or test widget hierarchies do not contain the expected three parent levels. All four affected call sites use the shared lookup.

Sequence diagram for null-safe MainWindow lookup

sequenceDiagram
    participant Caller
    participant CentralDocPage
    participant parentMainWindow
    participant QWidget
    participant MainWindow

    Caller->>CentralDocPage: isFullScreen()
    CentralDocPage->>parentMainWindow: parentMainWindow(this)
    loop Up to 3 levels while widget != nullptr
        parentMainWindow->>QWidget: parentWidget()
        QWidget-->>parentMainWindow: parent widget or nullptr
    end
    alt MainWindow found
        parentMainWindow->>MainWindow: isFullScreen()
        MainWindow-->>CentralDocPage: fullscreen state
    else Parent chain ends early or type mismatch
        parentMainWindow-->>CentralDocPage: nullptr
        CentralDocPage-->>Caller: false or return
    end

Loading

File-Level Changes

Change	Details	Files
Added a centralized, null-safe helper for resolving the owning MainWindow through the expected three-level widget parent chain.	Walk up to three parents while checking for null at every level. Perform the MainWindow dynamic cast only after completing a safe traversal. Return nullptr when the chain is incomplete or the target is not a MainWindow.	reader/uiframe/CentralDocPage.cpp
Applied the safe parent-chain lookup across all CentralDocPage fullscreen and sheet-count handling paths.	Replaced direct three-level parent dereferences in fullscreen status, entry, and exit methods. Replaced the lookup in sheet-count change handling while retaining existing null-aware behavior.	reader/uiframe/CentralDocPage.cpp

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