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.
(comment appended): In VS2015 (or above). Select Debug > Options > Debugging > General and unselect the "Enable Just My Code" option.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…