Hi Santhosh, thanks for the quick reply.
With the given example when i compare my dialog i see only difference is below in example dialog:
fileNameParameter="./hbgfileName"
fileReferenceParameter="./hbgfileReference"
when i try to add these params AEM is throwing "invalid name" error.
Moreover, i am not able to select more than one content fragments from Assets section of Toggle Side Panel at a time.