`corpus_properties()` crashes if corpus is not found

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This works ...

library(polmineR)
use("RcppCWB")
x <- corpus("REUTERS")
RcppCWB::corpus_properties(corpus = x@corpus, registry = x@registry_dir)

But if the corpus has not been loaded before, you see a crash.

RcppCWB::corpus_properties(corpus = x@corpus, registry = x@registry_dir)

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This is a minimal example. Run this line without loading RcppCWB before.

RcppCWB::corpus_properties(corpus = "REUTERS", registry = Sys.getenv("CORPUS_REGISTRY"))

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Fixed.