fix(surface): guard null parentContainer in ensureQmlContext

[deepin-wm]

parentContainer() can return null when the container has no parent. Calling qmlEngine() or setQmlEngine() on null leads to a crash.

Extracted from #1048. Ref: WM-29

Summary by Sourcery

Bug Fixes:

• Prevent crashes by avoiding qmlEngine and setQmlEngine calls when the parent container is null.

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: deepin-wm

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds a null check for the parent container before accessing its QML engine to prevent crashes when a surface container has no parent.

Flow diagram for ensureQmlContext null parent guard

flowchart TD
    A[ensureQmlContext]
    B[pc = parentContainer]
    C{pc is null?}
    D[return]
    E{"qmlEngine(pc) returns engine?"}
    F[pc.setQmlEngine]

    A --> B --> C
    C -- Yes --> D
    C -- No --> E
    E -- Yes --> F
    E -- No --> D

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File-Level Changes

Change	Details	Files
Guard against null parentContainer() in ensureQmlContext to avoid dereferencing a null pointer.	Store parentContainer() result in a local pointer variable for reuse. Return early from ensureQmlContext when the parent container is null. Reuse the cached parent container pointer when retrieving the QQmlEngine and when calling setQmlEngine.	src/surface/surfacecontainer.cpp

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