“Unrecognized selector sent to instance” when calling UIButton.addTarget in a class

Hi! I am trying to create a DropDownMenu class, but when I try to call addTarget to a UIButton this error comes up...

'unrecognized selector sent to instance 0x7fd167f06120' terminating with uncaught exception of type NSException'

I have been looking for the answer for hours now, and even on Stack Overflow, no one seems to have the answer. Any help is appreciated!

Here is the code for my DropDownMenu class...

Code Block 
class DropDownMenu: UIView {
	var main: UIButton! = UIButton(frame: CGRect(x: 0, y: 0, width: 46, height: 30))
	var view: UIView!
	var buttonTitles: [String]! = [""]
	var titleColor: UIColor! = UIColor.black
	var font: UIFont! = UIFont.systemFont(ofSize: 18)
	var buttonsBorderWidth: CGFloat! = 0
	var buttonsBorderColor: UIColor? = UIColor.white
	var buttonsCornerRadius: CGFloat! = 0
	var color: UIColor! = UIColor.clear
	var buttonsImageEdgeInsets: UIEdgeInsets? = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
	var images: [UIImage]? = nil
	var target: UIViewController!
	private var currentSelected: String? = nil
	private var optionsStack = UIStackView()
	init(main: UIButton) {
			self.main = main
			super.init(frame: CGRect())
	}
	func createDropDownMenu() {
			main.addTarget(target, action: #selector(DropDownMenu.openDropdown(_:)), for: .touchUpInside)
		
			print("Button Target?: \(main.allTargets), self.target: \(String(describing: target))")
		
			let mainFrame = main.frame
		
			optionsStack.frame = CGRect(x: mainFrame.minX, y: mainFrame.maxY, width: mainFrame.width, height: CGFloat(buttonTitles.count) * mainFrame.height)
			optionsStack.axis = .vertical
		
			view.addSubview(optionsStack)
						
			var y: CGFloat! = 0
		
			for title in buttonTitles {
					let button = UIButton(frame: CGRect(x: 0, y: y, width: mainFrame.width, height: mainFrame.height))
					button.setTitle(title, for: .normal)
					button.setTitleColor(titleColor, for: .normal)
					button.backgroundColor = color
					button.titleLabel?.font = font
					button.addTarget(target, action: #selector(DropDownMenu.onclick), for: .touchUpInside)
				
					y += mainFrame.height
				
					optionsStack.addArrangedSubview(button)
			}
		
			for button in optionsStack.arrangedSubviews {
					button.isHidden = true
					button.alpha = 0
			}
	}
	@objc private func openDropdown(_ sender: UIButton) {
			print("sender: \(String(describing: sender))")
			optionsStack.arrangedSubviews.forEach { (button) in
					UIView.animate(withDuration: 0.7) {
							button.isHidden = !button.isHidden
							button.alpha = button.alpha == 0 ? 1 : 0
							self.view.layoutIfNeeded()
					}
			}
	}
	@objc private func onclick(_ sender: UIButton) {
			let title = sender.titleLabel!.text
		
			print(title as Any)
		
			main.setTitle(title, for: .normal)
		
			optionsStack.arrangedSubviews.forEach { (button) in
					UIView.animate(withDuration: 0.7) {
							button.isHidden = true
							button.alpha = 0
					}
			}
	}
	required init?(coder: NSCoder) {
			fatalError("init(coder:) has not been implemented")
	}
}

Here is the code for the creation of the object in ViewContorller...

let main = UIButton(frame: CGRect(x: 50, y: 300, width: 80, height: 30))
main.layer.borderWidth = 1
main.setTitle("Grade", for: .normal)
main.titleLabel?.font = UIFont.boldSystemFont(ofSize: 20)
main.setTitleColor(UIColor.black, for: .normal)

let gradeDP = DropDownMenu(main: main)
gradeDP.buttonTitles = ["Title 1", "Title 2", "Title 3"]
gradeDP.color = UIColor.gray
gradeDP.target = self
gradeDP.titleColor = UIColor.white
gradeDP.view = infoBox

This is all of my code...