Issue #3556884 by alexpott, smustgrave:...

      $tree_children = [];

      foreach ($workspaces as $workspace_id => $workspace) {

        $tree_children[$workspace->parent->target_id][] = $workspace_id;

        $key = $workspace->hasParent() ? $workspace->parent->target_id : '';

        $tree_children[$key][] = $workspace_id;

      }

      // Keeps track of the parents we have to process, the last entry is used

      // for the next processing step. Top-level (root) workspace use NULL as

      // the parent, so we need to initialize the list with that value.

      $process_parents[] = NULL;

      // for the next processing step. Top-level (root) workspace use an empty

      // string as the parent, so we need to initialize the list with that

      // value.

      $process_parents[] = '';

      // Loops over the parent entities and adds its children to the tree array.

      // Uses a loop instead of a recursion, because it's more efficient.