how to handle null point exception?

Question

$(document).ready(function(){
try{
ExecuteOrDelayUntilScriptLoaded(downloads, "sp.js");
}
catch(e)
{
alert(JSON.stringify(e));
}
});
function downloads() {
var pagename = window.location.href.split('/').reverse()[0];
$.ajax({
url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getByTitle('DownloadLinks')/items?$filter=CategoryDownload eq '"+ pagename +"'&$orderby=Order_ID asc",
type: "GET",
headers: {
"accept": "application/json;odata=verbose",
"content-type": "application/json;odata=verbose",
},
success: DownloadLinksSucceeded,
error: DownloadLinksFailed
});
}
var results1;
var banner;
function DownloadLinksSucceeded(data) {
console.log(data);
results1 =data.d.results;

var htmlString="<div class='Category_heading'><h2 class='Category_heading1'>DOWNLOADS</h2></div>";

htmlString+="<ul class='no_bullet text'>";
for(var i=0; i< results1.length; i++){
var item1= results1[i];
htmlString += '<li class="image">';
htmlString += '<a nowrap class="link_tag" href="'+ results1[i].DownloadURL.Url +'">';
htmlString += results1[i].Title;

if(results1[i].Banner_Image.Url!=null)
{
var banner=results1[i].Banner_Image.Url;
htmlString +='<img class="baner_image" src="'+banner+'" alt="">';
}
else
{
alert('hello');
}

htmlString += '</a>';
htmlString+='</li>';
//htmlString += '<a class="link_tag" href="'+ results1[i].DownloadURL.Url +'">'+ results1[i].Title +'</a>';
}
htmlString+="</ul>";
$("#download1").html(htmlString);
}
function DownloadLinksFailed() {
alert('Sorry An Error Has Occurred!');
}