Check if cordova plugin file is installed

[michogar]

App doesn't check if the plugin is installed and throws an exception trying to get the database

mapbox-gl-cordova-offline.js:509 TypeError: Cannot read property 'applicationDirectory' of undefined
    at mapbox-gl-cordova-offline.js:591
    at new Promise (<anonymous>)
    at t.copyDatabaseFile (mapbox-gl-cordova-offline.js:591)
    at t.openDatabase (mapbox-gl-cordova-offline.js:591)
    at new t (mapbox-gl-cordova-offline.js:591)
    at exports.create (mapbox-gl-cordova-offline.js:209)
    at new t (mapbox-gl-cordova-offline.js:211)
    at t.addSource (mapbox-gl-cordova-offline.js:369)
    at e.addSource (mapbox-gl-cordova-offline.js:483)
    at mapbox-gl-cordova-offline.js:595
Evented.fire @ mapbox-gl-cordova-offline.js:509