Not sure what is the ask here - reason for NPE at checkNotNull() or otherwise why your code throws NPE or something else?
Preconditions.checkNotNull() is doing what it's supposed to do. If the passed value/object is null it would throw NPE.
Probably when you are trying to set property on node, it is not checked-out by same user or its already checked-out by different user or it has specific properties set that do not allow your operation or the underlying object is null.
Check why/how the passed value/object is null in your source code and check node properties that you're dealing with and fix it accordingly. Share appropriate code/screenshots, if applicable.