Fix panic when 404 is backed by a content file

Fixes #14283

hugolib/404_test.go

@@ -92,3 +92,24 @@ Not found.
 
 	b.AssertFileContent("public/404.html", `Not found. Updated.`)
 }
+
+func Test404Panic14283(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = "http://example.com/"
+-- layouts/all.html --
+All. {{ .Kind }}. {{ .Title }}|
+-- content/404/_index.md --
+---
+title: "404 branch"
+---
+This is the 404 branch.
+
+`
+
+	b := Test(t, files) // panic.
+
+	b.AssertFileContent("public/404/index.html", "All. section. 404 branch|")
+}

hugolib/content_map_page_assembler.go

@@ -198,6 +198,7 @@ func (a *allPagesAssembler) doCreatePages(prefix string, depth int) error {
 			p := newHomePageMetaSource()
 			treePages.InsertRaw(p.pathInfo.Base(), p)
 		}
+
 	}
 
 	getCascades := func(wctx *doctree.WalkContext[contentNode], s string) *page.PageMatcherParamsConfigs {

hugolib/doctree/nodeshifttree.go

@@ -625,7 +625,7 @@ func (r *NodeShiftTreeWalker[T]) toT(tree *NodeShiftTree[T], v T) (T, bool) {
 }
 
 func (r *NodeShiftTree[T]) Has(s string) bool {
-	_, ok := r.get(s)
+	_, ok := r.GetRaw(s)
 	return ok
 }