Xcode NSEException, signal SIGABRT can't figure it out

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!