Fix crash when adding output

include/sway/tree/container.h

@@ -351,6 +351,12 @@ bool container_is_floating_or_child(struct sway_container *container);
  */
 bool container_is_fullscreen_or_child(struct sway_container *container);
 
+/**
+ * Return the output which will be used for scale purposes.
+ * This is the most recently entered output.
+ */
+struct sway_output *container_get_effective_output(struct sway_container *con);
+
 void container_discover_outputs(struct sway_container *con);
 
 #endif

sway/tree/container.c

@@ -764,8 +764,7 @@ void container_damage_whole(struct sway_container *container) {
  * Return the output which will be used for scale purposes.
  * This is the most recently entered output.
  */
-static struct sway_output *container_get_effective_output(
-		struct sway_container *con) {
+struct sway_output *container_get_effective_output(struct sway_container *con) {
 	if (con->outputs->length == 0) {
 		return NULL;
 	}

sway/tree/view.c

@@ -889,8 +889,10 @@ void view_add_mark(struct sway_view *view, char *mark) {
 
 static void update_marks_texture(struct sway_view *view,
 		struct wlr_texture **texture, struct border_colors *class) {
-	struct sway_output *output =
-		view->swayc->outputs->items[view->swayc->outputs->length - 1];
+	struct sway_output *output = container_get_effective_output(view->swayc);
+	if (!output) {
+		return;
+	}
 	if (*texture) {
 		wlr_texture_destroy(*texture);
 		*texture = NULL;