Building success then "Thread 1: signal SIGABRT error" pops up

Question

I'm on the initial way of building a calculator. Currently, the code is doing nothing but printing the digits and Pi into the calculator's label when user taps them.

1) Che Code

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var display: UILabel! = nil

    var userIsInTheMiddleOfTypeing = false

    @IBAction func touchDigit(_ sender: UIButton){
        let digit = sender.currentTitle!
        if userIsInTheMiddleOfTypeing {
            let textCurrentlyInDisplay = display.text!
            display.text = textCurrentlyInDisplay + digit
        } else {
            display.text = digit
        }

        userIsInTheMiddleOfTypeing = true
    }

    @IBAction func performOperation(_ sender: UIButton) {
        userIsInTheMiddleOfTypeing = false
        if let methematicalSymbol = sender.currentTitle {
            if methematicalSymbol == "π" {
                display.text = String(M_PI) // M_PI
            }
        }
    }      
}

2) UI

The touchDigit function is linked to all the digit buttons as shown in the following figure. The display is the UILable while performOperaton is the PI button

Problem

When I build the code, Xcode first told me the building was success, However, before I could do anything, there is an error popped up as follow

Error Log (copied from the debug area)

2016-07-28 19:30:30.215343 Calculator[11671:208157] bundleid: com.Jeffery.Calculator, enable_level: 0, persist_level: 0, propagate_with_activity: 0
2016-07-28 19:30:30.218796 Calculator[11671:208157] Created DB, header sequence number = 260
2016-07-28 19:30:30.767300 Calculator[11671:208178] subsystem: com.apple.UIKit, category: HIDEvents, enable_level: 0, persist_level: 0, default_ttl: 0, info_ttl: 0, debug_ttl: 0, generate_symptoms: 0, enable_oversize: 0, privacy_setting: 0
2016-07-28 19:30:31.022078 Calculator[11671:208157] Created DB, header sequence number = 260
2016-07-28 19:30:31.350380 Calculator[11671:208157] subsystem: com.apple.BaseBoard, category: MachPort, enable_level: 0, persist_level: 0, default_ttl: 0, info_ttl: 0, debug_ttl: 0, generate_symptoms: 0, enable_oversize: 0, privacy_setting: 0
2016-07-28 19:30:31.388363 Calculator[11671:208159] subsystem: com.apple.FrontBoard, category: Common, enable_level: 0, persist_level: 0, default_ttl: 0, info_ttl: 0, debug_ttl: 0, generate_symptoms: 0, enable_oversize: 0, privacy_setting: 0
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

Answer 1

I figured it out my error, there are two connections attached with the PI button to the viewControler (I should delete one).

Answer 2

I think problem here: '@IBOutlet weak var display: UILabel! = nil'. When you do something with 'display' property you try do it with nil. Just try delete '= nil' after declarating property. Or you can override view controller init method, but as for me, in this situation it is bad way.