When I extend my UIViewController I'm getting NSLayoutConstraint error

Question

When I'm extending my UIViewController and calling the extended ViewController I'm getting : Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSLayoutConstraint for (null): Constraint must contain a first layout item' error. CGSize size = CGSizeMake(142, 200); [self.scrollView.subviews enumerateObjectsUsingBlock:^(UIView* subView, NSUInteger i, BOOL *stop) { subView.translatesAutoresizingMaskIntoConstraints = NO; [ViewHelper addWidthConstraint:subView width:size.width]; [ViewHelper addHeightConstraint:subView height:size.height]; if (i < self.scrollView.subviews.count - 1) { [ViewHelper addHorizontalConstraint:self.scrollView previouseView:subView nextView:(UIView*)self.scrollView.subviews[i + 1] spacer:8]; } [ViewHelper addEdgeConstraint:NSLayoutAttributeTop superview:self.scrollView subview:subView]; [ViewHelper addEdgeConstraint:NSLayoutAttributeBottom superview:self.scrollView subview:subView]; }]; [ViewHelper addEdgeConstraint:NSLayoutAttributeLeft superview:self.scrollView subview:self.scrollView.subviews.firstObject]; [ViewHelper addEdgeConstraint:NSLayoutAttributeRight superview:self.scrollView subview:self.scrollView.subviews.lastObject]; [ViewHelper addHeightConstraint:self.scrollView height:size.height]; Crashing at this line: [ViewHelper addEdgeConstraint:NSLayoutAttributeLeft superview:self.scrollView subview:self.scrollView.subviews.firstObject]; JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

The error indicates that you're attempting to add a constraint using a view which is nil. You should guard against one the self.scrollView.subviews.firstObject/lastObject being nil before attempt to add the constraints to them (by making sure the scrollView has any subviews). Here's an example: // make sure that the scrollView has some subviews before attempting to add layout constraints using the subviews if ([self.scrollView.subviews count] > 0) { [ViewHelper addEdgeConstraint:NSLayoutAttributeLeft superview:self.scrollView subview:self.scrollView.subviews.firstObject]; [ViewHelper addEdgeConstraint:NSLayoutAttributeRight superview:self.scrollView subview:self.scrollView.subviews.lastObject]; }