Fixed bug in array swapping function

ref #14

src/PdfCreator.ts

@@ -246,6 +246,10 @@ export class PdfCreator {
             return null;
         }
 
-        return [array[1], array[0]].concat(this.swapArrayPairs(array.slice(2)));
+        const rest = this.swapArrayPairs(array.slice(2))
+        if (!rest) {
+            return [array[1], array[0]]
+        }
+        return [array[1], array[0]].concat(rest);
     }
 }