Transaction fail on Ajax success on Document Ready using Cordova SQLite plugin

I know this is about asynchornous function after reading this: indexedDB: issue calling transaction function

However, I still couldn't figure out how to improve my code after reading all the asynchornous stuffs, here is my code:

index.html

<script src="cordova.js"></script>
<script src="js/db.js"></script>

<script>
    $( document ).ready(function() { 
        getLangText("zh");

        var thisShopId;

        $( "#list-view-section" ).hide();
        $( "#toggle-chart-view" ).hide();

        $.ajax(
        { 
          type: "GET",
          url: "http://mysite/myjson/",
          dataType: 'jsonp',
          xhrFields: {
               withCredentials: true
          },
          crossDomain: true,
          success: function(data){

          thisShopId = data[0].ShopId;

          addParam("ShopId", thisShopId);

          },
          error: function (xhr) {
                 alert(xhr.responseText);
          }
        }
       );
    });
</script>

db.js

document.addEventListener("deviceready", init, false);
document.addEventListener("touchstart", function() {}, false);

    var app = {};
    app.db = null;

    function init() {
        app.openDb();
        app.createTable();
    }

    app.openDb = function() {
        app.db = window.sqlitePlugin.openDatabase("BasicInfo");
    }

    app.createTable = function() {
        var db = app.db;
        db.transaction(function(tx) {
            tx.executeSql("CREATE TABLE IF NOT EXISTS workingparameters(ID INTEGER PRIMARY KEY ASC, name TEXT, value TEXT, added_on DATETIME)", [], app.onSuccess, app.onError);
        });
    }

    app.addParam = function(paramText, paramValue) {
        var db = app.db;
        db.transaction(function(tx) {
            var addedOn = new Date();
            tx.executeSql("INSERT INTO workingparameters(name, value, added_on) VALUES (?,?,?)",
                          [paramText, paramValue, addedOn],
                          app.onSuccess,
                          app.onError);
        });
    }

From time to time, the following problem occurs: "Uncaught TypeError: Cannot read property 'transaction' of null"

Please tell me how to improve my code, thanks.