Xcode Unit Test: Assert async throw/nothrow

I have an object which accepts an block as call back, like this

@interface MyObject

@property(nonatomic, copy) (void(^)()) callback;
- (void)trigger;

@end

When trigger is called, callback will be called asynchronously in 1 second, I want to write a test to make sure that if I set nil to callback, it won't crash due to calling a nil block.

How to do this?