private void computeConfigId() {
        if (parent == null) return;
        String parentConfigId = parent.getConfigIdPrefix();
        if (parentConfigId == null) return;
        String oldConfigId = configId;
        if (this instanceof AbstractConfigProducerRoot) {
            configId = "";
        } else {
            configId = parentConfigId + subId;
        }
        if (oldConfigId == null) return;
        if (!configId.equals(oldConfigId)) {
            throw new IllegalArgumentException("configId cannot change "+oldConfigId+" -> "+configId+" (invalid topology change)");
        }
    }
Returns the {@link ApplicationConfigProducerRoot} that is the parent of this sub-tree, or null
if this sub-tree has no Vespa parent.
config-model/src/main/java/com/yahoo/config/model/producer/AnyConfigProducer.java