Fix assert when setting default collapsed state of items

Would assert that the reconstruction phase is not completed. In this
case collapsing is modified as part of the construction, so the assert
is wrong. Access the collapsed state directly without using the getter
that does the assert check.

source/blender/editors/interface/views/tree_view.cc

@@ -516,7 +516,7 @@ bool AbstractTreeViewItem::set_collapsed(const bool collapsed)
   if (!is_collapsible()) {
     return false;
   }
-  if (collapsed == is_collapsed()) {
+  if (collapsed == !is_open_) {
     return false;
   }