parent invalid gives unclear error message

[FelixTheStudent]

Using a non-existent parent gives a cryptic error message. Minimal example:
simulated_umis %>% rule("T", "CD3E", ">", 1e-4, parent="oops") %>% plot_classes()

Note that the error only occurs through plot_classes. I think that's good behavior, we would not want to create obstacles when interactively developing rules.

The error is cryptic, please fix:

Error in if (current_parent == "..root..") { : argument is of length zero