C# in App_Web_qy3z2pcs.dll but was not handled

do null check in your code or fix the issue in dll and find out the root cause why that object is null in that dll some time depends on what you passing to the method in the dll. I am suggesting you generic cause of that particular exception.

we have bot used this dll file

this clearly says property or method is accessing through object but object is null in that dll

you should handle the exception with try catch in method in your project or in that .dll