If you are going to want to walk to a tile that's unwalkable you might run into these issues.
For exmaple the tree position is obviously unwalkable as it's blocked by the tree.
What you can do to solve this is either get a proper position or create a area around the position and walk to the area instead.
Try something like this:
Position treePosition = tree.getPosition();
Area treeArea = treePosition.getArea(3);
getWalking().webWalk(treeArea);