private InternalSource getChild(final String streamTypeName) {
            if (closed) {
                throw new RuntimeException("Closed");
            }

            if (streamTypeName == null) {
                return this;
            }

            return childMap.computeIfAbsent(streamTypeName, this::child);
        }
Gets the output stream for this stream target.
stroom-data/stroom-data-store-mock/src/main/java/stroom/data/store/mock/MockStore.java