Xcode NSEException, signal SIGABRT can't figure it out - swift

Question

I'm very new to Xcode, and I'm working on this code where, basically, a specific label appears depending on which button is pressed. To make it simpler (I don't know how many buttons and labels I'll have ultimately), I created arrays for labels and buttons (I dragged the buttons and labels to the code as Outlet Collection).
Anyway, here is the code:
import UIKit
class ViewController: UIViewController {
//initialize everything
//labels first
#IBOutlet var labels: [UILabel]!
//buttons now
#IBOutlet var buttons: [UIButton]!
//pictures next
#IBOutlet var pic: UIImageView!
//function for all buttons
#IBAction func button_action(_ sender: UIButton) {
let propertyToCheck = sender.tag
for i in 0...buttons.count-1{
if buttons[i].tag != propertyToCheck{
labels[i].isHidden = true
}
else{
labels[i].text = "The right text"
if labels[i].isHidden == true {
labels[i].isHidden = false
} else {
labels[i].isHidden = true
}
}
}
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
var index = 0
//initialize view
//labels
for label in labels{
label.isHidden = true
}
//pics
pic.isHidden = true
//associate action for every button
for button in buttons {
button.tag = index// setting tag, to identify button tapped in action method
print("Hello", button.tag, buttons.count)
button.addTarget(self, action: #selector(button_action(_:)), for: UIControlEvents.touchUpInside)
index = index + 1
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
I'm getting the following error: "libc++abi.dylib: terminating with uncaught exception of type NSException" and when I look at the AppDelegate, I find this: "Thread 1: signal SIGABRT". Here is the hierarchy:
Again, I'm very new, so I'm guessing it's an error in calling something that doesn't exist or something with the connections, but I can't figure out what it is! I already looked online for possible solutions, like cleaning and rebuilding the project, but nothing has worked so far.
Any suggestion would be really appreciated!