Minimal application to run https://stackoverflow.com/questions/50396073/xcode-9-3-1-ios-swift-4-1-uitextview-becomefirstresponder-thread-1-exc-bad-ac

AppDelegate.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        
        window = UIWindow(frame: UIScreen.main.bounds)
        window?.rootViewController = ErrorTestsController(nibName: nil, bundle: nil)
        window?.makeKeyAndVisible()
        
        // Override point for customization after application launch.
        return true
    }
}

class ErrorTestsController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        setupUIElements()
    }
    
    private func setupUIElements() {
        view.backgroundColor = UIColor.red
        view.addSubview(blankTextView)
        
        blankTextView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        blankTextView.widthAnchor.constraint(equalToConstant: 200.0).isActive = true
        blankTextView.heightAnchor.constraint(equalToConstant: 40.0).isActive = true
        
        blankTextViewBottomAnchor = blankTextView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: 0.0)
        blankTextViewBottomAnchor?.isActive = true
        // Whether I use safeAreaLayoutGuide or the regular view's bottomAnchor changes nothing
        
        // Add observers to move the blankTextView up when the keyboard appears
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShow(notification:)), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide(notification:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
        
        // add a UITapGestureRecognizer to make the blankTextView becomeFirstResponder and show the keyboard (will also trigger the above NotificationObserver)
        view.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(showAddTextView)))
    }
    
    // Define reference to bottomAnchor of the UITextView called "blankTextView" (using iOS 9.0 + NSLayoutConstraints)
    public var blankTextViewBottomAnchor: NSLayoutConstraint?
    
    public var blankTextView: UITextView = {
        let textview = UITextView(frame: .zero, textContainer: nil)
        textview.translatesAutoresizingMaskIntoConstraints = false
        textview.backgroundColor = UIColor.black
        return textview
    }()
    
    @objc private func showAddTextView() {
        blankTextView.becomeFirstResponder()
    }
    
    @objc private func keyboardWillShow(notification: NSNotification) {
        guard
            let keyboardFrame = (notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as AnyObject).cgRectValue,
            let keyboardDuration = (notification.userInfo?[UIKeyboardAnimationDurationUserInfoKey] as AnyObject).doubleValue
            else { return }
        
        blankTextViewBottomAnchor?.constant = -keyboardFrame.height
        
        UIView.animate(withDuration: keyboardDuration) {
            self.view.layoutIfNeeded()
        }
    }
    
    @objc private func keyboardWillHide(notification: NSNotification) {
        guard
            let keyboardDuration = (notification.userInfo?[UIKeyboardAnimationDurationUserInfoKey] as AnyObject).doubleValue
            else { return }
        
        blankTextViewBottomAnchor?.constant = 0.0
        
        UIView.animate(withDuration: keyboardDuration) {
            self.view.layoutIfNeeded()
        }
    }
}