public Shape expectNeighborShape() {
        if (neighborShape == null) {
            throw new ExpectationNotMetException("Neighbor does not exist: " + neighborShapeId, shape);
        }

        return neighborShape;
    }
Gets the neighbor shape or throws if it doesn't exist.

@return Returns the neighbor shape.
@throws ExpectationNotMetException if the neighbor is missing.
smithy-model/src/main/java/software/amazon/smithy/model/neighbor/Relationship.java