why i am getting errorr like this System.AggregateException

You could handle the exception directly so it would not crash your program (catching the AggregateException). You could also look at the Inner Exception, this will give you a more detailed explanation of what went wrong:

try {     // your code  } catch (AggregateException e) {  } 

As the message says, you have a task which threw an unhandled exception.

Turn on Break on All Exceptions (Debug, Exceptions) and rerun the program.
This will show you the original exception when it was thrown in the first place.