Because it's not freed.
Also, why not just use the value passed to the function instead? You can view, comment on, or merge this pull request online at:
https://github.com/geany/geany-plugins/pull/478
-- Commit Summary --
* treebrowser: Free `renderers` in `on_treeview_renamed()`.
-- File Changes --
M treebrowser/src/treebrowser.c (1)
-- Patch Links --
https://github.com/geany/geany-plugins/pull/478.patch https://github.com/geany/geany-plugins/pull/478.diff