错误提示 'null' 为空或不是对象
function ShowFLT(i) {
            var lbmc ;
            var treePic;
            for (x = 1; x <= 12; x++) {
                if (i == x) {
                    lbmc = document.getElementById('LM' + i);
                    treePic = document.getElementById('treePic' + i);
                    if (lbmc.style.display == "none") {
                        treePic.className = 'lmenua';
                        lbmc.style.display = '';
                    }
                    else {
                        treePic.className = 'lmenub';
                        lbmc.style.display = 'none';
                    }
                }
                else {
                        lbmc = document.getElementById('LM' + x);
                        treePic = document.getElementById('treePic' + x);
                        treePic.className = 'lmenub';
                        lbmc.style.display = 'none';
                   
                }
            }
        }

最后两句错误提示 'null' 为空或不是对象，什么原因，求解.