fixed potential null pointer
fixed potential null pointer

sitewaerts/cordova-plugin-document-viewer,bcalvez/cordova-plugin-document-viewer,bcalvez/cordova-plugin-document-viewer,micaTewe/cordova-plugin-document-viewer,sitewaerts/cordova-plugin-document-viewer,sitewaerts/cordova-plugin-document-viewer,sitewaerts/cordova-plugin-document-viewer,micaTewe/cordova-plugin-document-viewer