         let maxHeight = screenSize.height - keyboardHeight
         
         // for avoiding constraint error
-        buttonAreaScrollViewHeightConstraint.constant = 0.0
+        buttonAreaScrollViewHeightConstraint?.constant = 0.0
         
         // AlertView Height Constraint
         var alertViewHeight = textAreaHeight + buttonAreaHeight

    
    // ContainerView
    private var containerView = UIView()
    private var containerViewBottomSpaceConstraint: NSLayoutConstraint!
    
    // AlertView
    public var alertView = UIView()
    public var alertViewBgColor = UIColor(red:239/255, green:240/255, blue:242/255, alpha:1.0)
    private var alertViewWidth: CGFloat = 270.0
    private var alertViewHeightConstraint: NSLayoutConstraint!
    private var alertViewPadding: CGFloat = 15.0
    private var innerContentWidth: CGFloat = 240.0
    private let actionSheetBounceHeight: CGFloat = 20.0

    
    // ButtonAreaScrollView
    private var buttonAreaScrollView = UIScrollView()
    private var buttonAreaScrollViewHeightConstraint: NSLayoutConstraint?
    private var buttonAreaHeight: CGFloat = 0.0
    
    // ButtonAreaView
