Hello Vikas,
Unfortunately the information and code given below isn't enough for us to debug your issue. What we can tell you is ErrorDesc does not contain a string so the method lastIndexOf does not exist. What may be helpful is to look further up in the code you have written to determine when ErrorDesc was created and assigned. If this does not help please feel free to include some more information about the error and your code.
Regards,
Plum Support