The method end() does not exist on Symfony\Component\Config...der\NodeParentInterface. It seems like you code against a sub-type of said class. However, the method does not exist in Symfony\Component\Config...ion\Builder\TreeBuilder. Are you sure you never get one of those?
Ignorable by Annotation
If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation