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null reference on ZipStorer.Close()
If you call the Close() method within a using clause you get a null reference.
using (ZipStorer zipFile = ZipStorer.Create(name, ""))
zipFile.AddStream(ZipStorer.Compression.Deflate, "test.txt", instream, DateTime.Now, "");
in the Close() method first check if stream is null:
if (this.ZipFileStream == null)