only render in double page mode once both pages have been preloaded

app/comic-book.js

@@ -95,7 +95,14 @@ class ComicBook extends EventEmitter {
     let args = [ page ]
 
     if (this.options.doublePage) {
-      args.push(this.pages.get(pageIndex + 1))
+      let page2Index = pageIndex + 1
+      let page2 = this.pages.get(page2Index)
+
+      if (page2Index <= (this.pages.size - 1) && !page2) {
+        return this.preload()
+      }
+
+      args.push(page2)
 
       if (this.options.rtl) {
         args.reverse()

dist/comicbook.js

@@ -123,7 +123,14 @@ var ComicBook = (function (_EventEmitter) {
       var args = [page];
 
       if (this.options.doublePage) {
-        args.push(this.pages.get(pageIndex + 1));
+        var page2Index = pageIndex + 1;
+        var page2 = this.pages.get(page2Index);
+
+        if (page2Index <= this.pages.size - 1 && !page2) {
+          return this.preload();
+        }
+
+        args.push(page2);
 
         if (this.options.rtl) {
           args.reverse();