title: Cannot modify constraints for UIToolbar managed by a UINavigationController
body: <p>I am adding NSContraints to a Toolbar of of <code>UINavigationController</code> and I get this error:</p>
  
  <p>Cannot modify constraints for <code>UIToolbar</code> managed by a <code>UINavigationController</code></p>
  
  <pre><code>In ViewDidLoad:
    self.navigationController.toolbarHidden = NO;
  [self.navigationController.toolbar addSubview:_labelName];
  [self.navigationController.toolbar addSubview:_labelAddress];
  </code></pre>
  
  <p>Then I call the method below:</p>
  
  <pre><code>NSLayoutConstraint *nameRight = [NSLayoutConstraint constraintWithItem:_labelName attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeRight multiplier:1 constant:20];
  NSLayoutConstraint *nameLeft = [NSLayoutConstraint constraintWithItem:_labelName attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeLeft multiplier:1 constant:20];
  NSLayoutConstraint *nameTop = [NSLayoutConstraint constraintWithItem:_labelName attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeTop multiplier:1 constant:0];
  NSLayoutConstraint *nameHeight = [NSLayoutConstraint constraintWithItem:_labelName attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeHeight multiplier:.5 constant:0];
  
  NSLayoutConstraint *addressRight = [NSLayoutConstraint constraintWithItem:_labelAddress attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeRight multiplier:1 constant:20];
  NSLayoutConstraint *addressLeft = [NSLayoutConstraint constraintWithItem:_labelAddress attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeLeft multiplier:1 constant:20];
  NSLayoutConstraint *addressTop = [NSLayoutConstraint constraintWithItem:_labelAddress attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:_labelName attribute:NSLayoutAttributeBottom multiplier:1 constant:0];
  NSLayoutConstraint *addressHeight = [NSLayoutConstraint constraintWithItem:_labelAddress attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.navigationController.toolbar attribute:NSLayoutAttributeHeight multiplier:.5 constant:0];
  
  
  [self.navigationController.toolbar addConstraints:@[nameRight, nameLeft, nameTop, nameHeight, addressRight, addressLeft, addressTop, addressHeight]];
  </code></pre>
  
  <p>Should I create a <code>UIView</code> inside the toolbar of the navigation controller? and put the contraints inside it?</p>