code: protected final JsonNode getRequiredNode(JsonNode tree, String fieldName) {
  		Assert.notNull(tree, "Tree must not be null");
  		JsonNode node = tree.get(fieldName);
  		Assert.state(node != null && !(node instanceof NullNode),
  				() -> "Missing JSON field '" + fieldName + "'");
  		return node;
  	}
docs: Helper method to return a {@link JsonNode} from the tree.
  @param tree the source tree
  @param fieldName the field name to extract
  @return the {@link JsonNode}