treelistmodel: Be safer during collapsing

While we are collapsing a subtree, some signal handlers may not be
disconnected while we are doing this. By adding this check and not
giving those nodes no longer access to the model, we can stop it from
modifying it while we are trying to collapse stuff.

Fixes some crashes in gnome-builder.

gtk/gtktreelistmodel.c

@@ -101,8 +101,15 @@ static GParamSpec *properties[NUM_PROPERTIES] = { NULL, };
 static GtkTreeListModel *
 tree_node_get_tree_list_model (TreeNode *node)
 {
-  for (; !node->is_root; node = node->parent)
-    { }
+  if (node->is_root)
+    return node->list;
+
+  for (node = node->parent; !node->is_root; node = node->parent)
+    {
+      /* This can happen during collapsing of a parent node */
+      if (node->children == NULL)
+        return NULL;
+    }
 
   return node->list;
 }
@@ -316,6 +323,9 @@ gtk_tree_list_model_items_changed_cb (GListModel *model,
   guint i, tree_position, tree_removed, tree_added, n_local;
 
   self = tree_node_get_tree_list_model (node);
+  if (self == NULL)
+    return;
+
   n_local = g_list_model_get_n_items (model) - added + removed;
 
   if (position < n_local)
@@ -1180,6 +1190,8 @@ gtk_tree_list_row_set_expanded (GtkTreeListRow *self,
     return;
 
   list = tree_node_get_tree_list_model (self->node);
+  if (list == NULL)
+    return;
 
   if (expanded)
     {
@@ -1255,6 +1267,9 @@ gtk_tree_list_row_is_expandable (GtkTreeListRow *self)
     return TRUE;
 
   list = tree_node_get_tree_list_model (self->node);
+  if (list == NULL)
+    return FALSE;
+
   model = tree_node_create_model (list, self->node);
   if (model)
     {