fix: harden tree root cwd fetch (#2557)

Co-authored-by: Alexander Courtis <alex@courtis.org>

Author: kezhenxu94

lua/nvim-tree/core.lua

@@ -28,7 +28,7 @@ function M.get_explorer()
 end
 
 function M.get_cwd()
-  return TreeExplorer.absolute_path
+  return TreeExplorer and TreeExplorer.absolute_path
 end
 
 function M.get_nodes_starting_line()