Hi Ravi,
I found the solution to the issue. Commenting the line jQuery.sap.declare("Application"); doesn't help, instead we need to add the following line after that:
jQuery.sap.require("sap.ui.app.Application");
i think that the sap.ui.app.Application module was not loaded earlier which caused this issue, but when we add this jQuery.sap.require statement, system checks if this module is already loaded into the memory, if not it loads the module.
Thanks anyways. Cheers.