`FileSystemDock` Fix crash when renaming a file in split mode

(cherry picked from commit 25ce459fc5e324227b8ef3b776c8c7e95a3c71a9)

editor/filesystem_dock.cpp

@@ -1450,14 +1450,13 @@ void FileSystemDock::_folder_removed(String p_folder) {
 
 void FileSystemDock::_rename_operation_confirm() {
 	String new_name = rename_dialog_text->get_text().strip_edges();
-	String old_name = tree->get_selected()->get_text(0);
 	if (new_name.length() == 0) {
 		EditorNode::get_singleton()->show_warning(TTR("No name provided."));
 		return;
 	} else if (new_name.find("/") != -1 || new_name.find("\\") != -1 || new_name.find(":") != -1) {
 		EditorNode::get_singleton()->show_warning(TTR("Name contains invalid characters."));
 		return;
-	} else if (to_rename.is_file && old_name.get_extension() != new_name.get_extension()) {
+	} else if (to_rename.is_file && to_rename.path.get_extension() != new_name.get_extension()) {
 		if (!EditorFileSystem::get_singleton()->get_valid_extensions().find(new_name.get_extension())) {
 			EditorNode::get_singleton()->show_warning(TTR("This file extension is not recognized by the editor.\nIf you want to rename it anyway, use your operating system's file manager.\nAfter renaming to an unknown extension, the file won't be shown in the editor anymore."));
 			return;