prevent NPE when discriminator is not set

[mak-42]

This code fails if someone forget to set discriminator in SiftingAppender:

        if (discriminator == null) {
            addError("Missing discriminator. Aborting");
            errors++;
        }
        if (!discriminator.isStarted()) {
            addError("Discriminator has not started successfully. Aborting");
            errors++;
        }

I fixed it and added the test to check it.