LibGUI: Make FilteringProxyModel reference-count its parent model

Before this change, the destructor of FilteringProxyModel
would crash if the parent model had been destroyed earlier.

This unifies the behaviour of FilteringProxyModel with
SortingProxyModel in this respect.

Userland/Libraries/LibGUI/FilteringProxyModel.cpp

@@ -30,7 +30,7 @@ int FilteringProxyModel::column_count(ModelIndex const& index) const
     if ((size_t)index.row() > m_matching_indices.size() || index.row() < 0)
         return 0;
 
-    return m_model.column_count(m_matching_indices[index.row()]);
+    return m_model->column_count(m_matching_indices[index.row()]);
 }
 
 Variant FilteringProxyModel::data(ModelIndex const& index, ModelRole role) const
@@ -55,12 +55,12 @@ void FilteringProxyModel::filter()
     m_matching_indices.clear();
 
     Function<void(ModelIndex&)> add_matching = [&](ModelIndex& parent_index) {
-        for (auto i = 0; i < m_model.row_count(parent_index); ++i) {
-            auto index = m_model.index(i, 0, parent_index);
+        for (auto i = 0; i < m_model->row_count(parent_index); ++i) {
+            auto index = m_model->index(i, 0, parent_index);
             if (!index.is_valid())
                 continue;
 
-            auto filter_matches = m_model.data_matches(index, m_filter_term);
+            auto filter_matches = m_model->data_matches(index, m_filter_term);
             bool matches = filter_matches == TriState::True;
             if (filter_matches == TriState::Unknown) {
                 auto data = index.data();
@@ -100,12 +100,12 @@ ModelIndex FilteringProxyModel::map(ModelIndex const& index) const
 
 bool FilteringProxyModel::is_searchable() const
 {
-    return m_model.is_searchable();
+    return m_model->is_searchable();
 }
 
 Vector<ModelIndex> FilteringProxyModel::matches(StringView searching, unsigned flags, ModelIndex const& index)
 {
-    auto found_indices = m_model.matches(searching, flags, index);
+    auto found_indices = m_model->matches(searching, flags, index);
     for (size_t i = 0; i < found_indices.size(); i++)
         found_indices[i] = map(found_indices[i]);
     return found_indices;

Userland/Libraries/LibGUI/FilteringProxyModel.h

@@ -17,14 +17,14 @@ namespace GUI {
 class FilteringProxyModel final : public Model
     , public ModelClient {
 public:
-    static ErrorOr<NonnullRefPtr<FilteringProxyModel>> create(Model& model)
+    static ErrorOr<NonnullRefPtr<FilteringProxyModel>> create(NonnullRefPtr<Model> model)
     {
-        return adopt_nonnull_ref_or_enomem(new (nothrow) FilteringProxyModel(model));
+        return adopt_nonnull_ref_or_enomem(new (nothrow) FilteringProxyModel(move(model)));
     }
 
     virtual ~FilteringProxyModel() override
     {
-        m_model.unregister_client(*this);
+        m_model->unregister_client(*this);
     };
 
     virtual int row_count(ModelIndex const& = ModelIndex()) const override;
@@ -44,13 +44,13 @@ protected:
 
 private:
     void filter();
-    explicit FilteringProxyModel(Model& model)
-        : m_model(model)
+    explicit FilteringProxyModel(NonnullRefPtr<Model> model)
+        : m_model(move(model))
     {
-        m_model.register_client(*this);
+        m_model->register_client(*this);
     }
 
-    Model& m_model;
+    NonnullRefPtr<Model> m_model;
 
     // Maps row to actual model index.
     Vector<ModelIndex> m_matching_indices;