Help+LibManual: Make the children accessor fallible

This is convenient for the section node which might compute children
on the fly.

Userland/Applications/Help/ManualModel.cpp

@@ -109,8 +109,11 @@ GUI::ModelIndex ManualModel::index(int row, int column, const GUI::ModelIndex& p
     if (!parent_index.is_valid())
         return create_index(row, column, Manual::sections[row].ptr());
     auto* parent = static_cast<Manual::Node const*>(parent_index.internal_data());
-    auto* child = &parent->children()[row];
-    return create_index(row, column, child);
+    auto const children = parent->children();
+    if (children.is_error())
+        return {};
+    auto child = children.value()[row];
+    return create_index(row, column, child.ptr());
 }
 
 GUI::ModelIndex ManualModel::parent_index(const GUI::ModelIndex& index) const
@@ -128,8 +131,12 @@ GUI::ModelIndex ManualModel::parent_index(const GUI::ModelIndex& index) const
                 return create_index(row, 0, parent);
         VERIFY_NOT_REACHED();
     }
-    for (size_t row = 0; row < parent->parent()->children().size(); row++) {
-        Manual::Node* child_at_row = &parent->parent()->children()[row];
+    auto maybe_children = parent->parent()->children();
+    if (maybe_children.is_error())
+        return {};
+    auto children = maybe_children.release_value();
+    for (size_t row = 0; row < children.size(); row++) {
+        Manual::Node* child_at_row = children[row];
         if (child_at_row == parent)
             return create_index(row, 0, parent);
     }
@@ -141,7 +148,10 @@ int ManualModel::row_count(const GUI::ModelIndex& index) const
     if (!index.is_valid())
         return static_cast<int>(Manual::sections.size());
     auto* node = static_cast<Manual::Node const*>(index.internal_data());
-    return node->children().size();
+    auto maybe_children = node->children();
+    if (maybe_children.is_error())
+        return 0;
+    return static_cast<int>(maybe_children.value().size());
 }
 
 int ManualModel::column_count(const GUI::ModelIndex&) const

Userland/Libraries/LibManual/Node.h

@@ -20,7 +20,7 @@ class Node : public RefCounted<Node> {
 public:
     virtual ~Node() = default;
 
-    virtual NonnullRefPtrVector<Node>& children() const = 0;
+    virtual ErrorOr<Span<NonnullRefPtr<Node>>> children() const = 0;
     virtual Node const* parent() const = 0;
     virtual ErrorOr<String> name() const = 0;
     virtual bool is_page() const { return false; }

Userland/Libraries/LibManual/PageNode.cpp

@@ -16,10 +16,10 @@ Node const* PageNode::parent() const
     return m_section.ptr();
 }
 
-NonnullRefPtrVector<Node>& PageNode::children() const
+ErrorOr<Span<NonnullRefPtr<Node>>> PageNode::children() const
 {
     static NonnullRefPtrVector<Node> empty_vector;
-    return empty_vector;
+    return empty_vector.span();
 }
 
 ErrorOr<String> PageNode::path() const

Userland/Libraries/LibManual/PageNode.h

@@ -23,7 +23,7 @@ public:
     {
     }
 
-    virtual NonnullRefPtrVector<Node>& children() const override;
+    virtual ErrorOr<Span<NonnullRefPtr<Node>>> children() const override;
     virtual Node const* parent() const override;
     virtual ErrorOr<String> name() const override { return m_page; };
     virtual bool is_page() const override { return true; }

Userland/Libraries/LibManual/SectionNode.h

@@ -23,10 +23,10 @@ public:
     {
     }
 
-    virtual NonnullRefPtrVector<Node>& children() const override
+    virtual ErrorOr<Span<NonnullRefPtr<Node>>> children() const override
     {
-        MUST(reify_if_needed());
-        return m_children;
+        TRY(reify_if_needed());
+        return m_children.span();
     }
 
     virtual Node const* parent() const override { return nullptr; }