Optional closures in Swift 4

I need a closure method executed on demand.

In Swift 3 I used to do something like this:

// Declare closure
var checksPerformed: ((Void) -> Void)? // Declaration

// Call when needed
checksPerformed?() 

//Only executes when checksPerformed is called
checksPerformed = { _ in 
    // do stuff here
}

In Swift 4 this is no longer the case.

And after fixing this warning nothing works as before. What is the new way of doing this?

If I'm updating the declaration to: var checksPerformed: (() -> ())? I'm getting