It doesn't happen with the package's example file but does with Genome Analysis ToolKit's gnomAD allele frequency file.
populationFreq <- readVcf("af-only-gnomad.hg38.vcf.gz")
expand(populationFreq)
Error in array(vec, c(idxlen, xcols, 2L)) :
'data' must be of a vector type, was 'NULL'
> traceback()
6: array(vec, c(idxlen, xcols, 2L))
5: .expandAD(gvarR, length(idx), ncol(x))
4: .expandGeno(x, hdr, elt, idx)
It doesn't happen with the package's example file but does with Genome Analysis ToolKit's gnomAD allele frequency file.