introspection: This patch fixes nullable return values fixes for the following symbol...

introspection: This patch fixes nullable return values fixes for the following symbols in gtk

  gtk_accel_group_query
  gtk_accel_group_from_accel_closure
  gtk_accel_label_get_accel_widget
  gtk_accessible_get_widget
  gtk_actionable_get_action_name
  gtk_app_chooser_get_app_info
  gtk_app_chooser_button_get_heading
  gtk_app_chooser_dialog_get_heading
  gtk_application_get_window_by_id
  gtk_assistant_get_nth_page
  gtk_binding_set_find
  gtk_builder_get_object
  gtk_builder_lookup_callback_symbol
  gtk_builder_get_application
  gtk_button_get_image
  gtk_cell_area_get_focus_from_sibling
  gtk_cell_renderer_start_editing
  gtk_cell_view_get_model
  gtk_cell_view_get_displayed_row
  gtk_clipboard_get_owner
  gtk_container_get_focus_child
  gtk_container_get_focus_vadjustment
  gtk_container_get_focus_hadjustment
  gtk_dialog_get_widget_for_response
  gtk_drag_get_source_widget
  gtk_drag_dest_get_target_list
  gtk_drag_source_get_target_list
  gtk_entry_completion_get_model
  gtk_entry_completion_compute_prefix
  gtk_expander_get_label_widget
  gtk_file_chooser_get_filename
  gtk_file_chooser_get_current_folder
  gtk_file_chooser_get_uri
  gtk_file_chooser_get_current_folder_uri
  gtk_file_chooser_get_preview_widget
  gtk_file_chooser_get_preview_file
  gtk_file_chooser_get_preview_filename
  gtk_file_chooser_get_preview_uri
  gtk_file_chooser_get_extra_widget
  gtk_file_chooser_get_filter
  gtk_file_chooser_native_get_accept_label
  gtk_file_chooser_native_get_cancel_label
  gtk_file_filter_get_name
  gtk_font_chooser_get_font_family
  gtk_font_chooser_get_font_face
  gtk_font_chooser_get_font
  gtk_font_chooser_get_font_desc
  gtk_font_chooser_get_font_map
  gtk_frame_get_label
  gtk_gesture_get_device
  gtk_gesture_get_window
  gtk_gl_area_get_error
  gtk_header_bar_get_title
  gtk_header_bar_get_subtitle
  gtk_header_bar_get_custom_title
  gtk_icon_info_get_filename
  gtk_icon_view_get_path_at_pos
  gtk_icon_view_get_model
  gtk_image_get_pixbuf
  gtk_image_get_animation
  gtk_label_get_mnemonic_widget
  gtk_label_get_attributes
  gtk_check_version
  gtk_menu_button_get_popup
  gtk_menu_button_get_menu_model
  gtk_menu_button_get_align_widget
  gtk_menu_button_get_popover
  gtk_menu_item_get_submenu
  gtk_menu_item_get_accel_path
  gtk_native_dialog_get_title
  gtk_native_dialog_get_transient_for
  gtk_notebook_get_nth_page
  gtk_notebook_get_tab_label_text
  gtk_notebook_get_menu_label
  gtk_notebook_get_menu_label_text
  gtk_notebook_get_group_name
  gtk_notebook_get_action_widget
  gtk_offscreen_window_get_surface
  gtk_offscreen_window_get_pixbuf
  gtk_paned_get_child1
  gtk_paned_get_child2
  gtk_places_sidebar_get_location
  gtk_places_sidebar_get_nth_bookmark
  gtk_plug_get_socket_window
  gtk_popover_get_default_widget
  gtk_progress_bar_get_text
  gtk_recent_filter_get_name
  gtk_recent_manager_lookup_item
  gtk_settings_get_default
  gtk_socket_get_plug_window
  gtk_stack_sidebar_get_stack
  gtk_stack_switcher_get_stack
  gtk_style_context_get_section
  gtk_style_context_get_parent
  gtk_style_context_get_frame_clock
  gtk_test_find_widget
  gtk_text_buffer_get_mark
  gtk_text_tag_table_lookup
  gtk_text_view_get_tabs
  gtk_text_view_toggle_cursor_visible
  gtk_text_view_get_window
  gtk_toolbar_get_nth_item
  gtk_tool_button_get_label
  gtk_tool_button_get_icon_name
  gtk_tool_button_get_label_widget
  gtk_tool_button_get_icon_widget
  gtk_tool_palette_get_drop_item
  gtk_tool_palette_get_drop_group
  gtk_tree_model_filter_convert_child_path_to_path
  gtk_tree_model_filter_convert_path_to_child_path
  gtk_tree_model_sort_convert_child_path_to_path
  gtk_tree_model_sort_convert_path_to_child_path
  gtk_tree_view_get_column
  gtk_tree_view_get_bin_window
  gtk_tree_view_column_get_widget
  gtk_tree_view_column_get_tree_view
  gtk_widget_get_frame_clock
  gtk_window_group_get_current_device_grab
  GtkTextBufferSerializeFunc