#vulnerable code 
                                @Override
    public void renameMailbox(MailFolder existingFolder, String newName) throws FolderException {
        HierarchicalFolder toRename = (HierarchicalFolder) existingFolder;
        HierarchicalFolder parent = toRename.getParent();

        int idx = newName.lastIndexOf(ImapConstants.HIERARCHY_DELIMITER_CHAR);
        String newFolderName;
        String newFolderPathWithoutName;
        if (idx > 0) {
            newFolderName = newName.substring(idx + 1);
            newFolderPathWithoutName = newName.substring(0, idx);
        } else {
            newFolderName = newName;
            newFolderPathWithoutName = "";
        }

        if (parent.getName().equals(newFolderPathWithoutName)) {
            // Simple rename
            toRename.setName(newFolderName);
        } else {
            // Hierarchy change
            parent.removeChild(toRename);
            HierarchicalFolder userFolder = getInboxOrUserRootFolder(toRename);
            String[] path = newName.split('\\' + ImapConstants.HIERARCHY_DELIMITER);
            HierarchicalFolder newParent = userFolder;
            for (int i = 0; i < path.length - 1; i++) {
                newParent = newParent.getChild(path[i]);
            }
            toRename.moveToNewParent(newParent);
            toRename.setName(newFolderName);
        }
    } 
                            #location 27 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void renameMailbox(MailFolder existingFolder, String newName) {
        HierarchicalFolder toRename = (HierarchicalFolder) existingFolder;
        HierarchicalFolder parent = toRename.getParent();

        int idx = newName.lastIndexOf(ImapConstants.HIERARCHY_DELIMITER_CHAR);
        String newFolderName;
        String newFolderPathWithoutName;
        if (idx > 0) {
            newFolderName = newName.substring(idx + 1);
            newFolderPathWithoutName = newName.substring(0, idx);
        } else {
            newFolderName = newName;
            newFolderPathWithoutName = "";
        }

        if (parent.getName().equals(newFolderPathWithoutName)) {
            // Simple rename
            toRename.setName(newFolderName);
        } else {
            // Hierarchy change
            parent.removeChild(toRename);
            HierarchicalFolder userFolder = getInboxOrUserRootFolder(toRename);
            String[] path = newName.split('\\' + ImapConstants.HIERARCHY_DELIMITER);
            HierarchicalFolder newParent = userFolder;
            for (int i = 0; i < path.length - 1; i++) {
                newParent = newParent.getChild(path[i]);
            }
            toRename.moveToNewParent(newParent);
            toRename.setName(newFolderName);
        }
    }