I have created a height constraint and i am assigning a value to it according to the screen size. But while assigning I get the error `Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value` on the line `heightConstraint.constant = Height`

```
var heightConstraint: NSLayoutConstraint!
let Height =((intra_acc_send_b.frame.origin.y+intra_acc_send_b.frame.size.height)+20)

print ("\nheight%%%%%%%%%%%%%%%% ") ; print(Height); print(" %%%%%%%%%%%%%%%%%\n");
heightConstraint.constant = Height 
self.scrollView.contentSize = CGSize(width: Width,  height: heightConstraint.constant )

```

Kindly help me to find out the solution.