fix(v3): guard nil AllowDarkModeForWindow to avoid startup panic

[roachadam]

Description

Fixes a nil-pointer panic on startup when a window uses a Dark (or system-dark) native title bar on Windows builds that do not load the dark-mode uxtheme APIs — e.g. Windows 10 1809 / Windows Server 2019 (build 17763).

pkg/w32/theme.go loads AllowDarkModeForWindow (and siblings) only on build >= 18334, so on 1809 the package-level w32.AllowDarkModeForWindow is nil. (*windowsWebviewWindow).run called it unguarded in the Dark and dark SystemDefault branches (webview_window_windows.go), while w32.SetMenuTheme` already guards the identical call.

The fix extracts the call into a small applyDarkMode helper that nil-guards it (mirroring the guard w32.SetMenuTheme already uses) and calls it from both branches, so the window degrades gracefully (default title bar) instead of panicking. No behavior change on builds that load the procs; no new dependencies.

Fixes #5792

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Built a minimal v3 app with Windows: application.WindowsWindow{Theme: application.Dark} and ran it on Windows Server 2019 (build 17763): before, it panicked on startup; after, the window opens normally.
Reproduction repo: https://github.com/roachadam/wails-1809-dark-title-bar-poc.

• Windows
• macOS
• Linux

Test Configuration

Wails: v3.0.0-alpha2.117
OS:    Windows Server 2019 Standard Evaluation, 1809 (Build 17763)
Arch:  amd64
WebView2 runtime: 150.0.4078.65

Wails is not installed on this VM, so there is no wails3 doctor output. Reproduced with a minimal app instead: my existing app (built on v3.0.0-alpha.68) failed to start on Windows Server 2019 Standard Evaluation (build 17763). I reproduced the panic in isolation using the POC above built against the tagged v3.0.0-alpha2.117 release, then rebuilt the POC against this fix and confirmed the window opens normally.

Checklist:

• (v2 only) I have updated website/src/pages/changelog.mdx with details of this PR (v3 changelog entries are added automatically)
• My code follows the general coding style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Summary by CodeRabbit

• Bug Fixes
  • Fixed a Windows startup crash that could occur when using a dark or system-dark window theme.
  • Improved compatibility with Windows versions where dark-mode system APIs are unavailable.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 18049280-9c7c-4913-a831-0f4da4948c31

📥 Commits

Reviewing files that changed from the base of the PR and between 107e42c and eda4f52.

📒 Files selected for processing (2)

• v3/UNRELEASED_CHANGELOG.md
• v3/pkg/application/webview_window_windows.go

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Walkthrough

Windows dark-theme initialization now uses a nil-safe helper before invoking the optional dark-mode API. Both dark system-default and explicit dark themes avoid startup panics on builds without that API, and the fix is documented in the unreleased changelog.

Changes

Windows dark-mode startup fix

Layer / File(s)	Summary
Nil-safe dark-mode theme setup v3/pkg/application/webview_window_windows.go, v3/UNRELEASED_CHANGELOG.md	Dark system-default and explicit dark themes call applyDarkMode, which checks whether AllowDarkModeForWindow is available before invoking it. The changelog records the fix.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• wailsapp/wails#5790: Addresses the same Windows startup panic involving the unavailable dark-mode API.

Suggested labels: Bug, v3, size:XS

Suggested reviewers: wayneforrest, taliesin-ai, mbaklor

Poem

A rabbit found a dark-mode snare,
And guarded the Windows call with care.
No nil hop, no startup fright,
Just graceful themes from day to night.
Thump thump! Fixed and bright.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main fix: guarding nil AllowDarkModeForWindow to prevent a startup panic.
Description check	✅ Passed	The description covers the fix, issue number, testing, configuration, and change type well enough for this repository.
Linked Issues check	✅ Passed	The code matches #5792 by nil-guarding dark-mode window setup and preserving behavior on supported Windows builds.
Out of Scope Changes check	✅ Passed	The changes stay focused on the reported Windows dark-mode panic and the related changelog entry.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (2.12.2)

level=error msg="[linters_context] typechecking error: pattern ./...: directory prefix . does not contain modules listed in go.work or their selected dependencies"

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