XSSFSheet.CopySheet - fix null reference error

[Bykiev]

Closes #346

[tonyqus]

Do you figure out why clonedSheet.AddRelation(rel.Id, r) throws exception?

[Bykiev]

I believe it's a copy-paste error, but still not 100% sure in my fix, so, it should be reviewed carefully.

npoi/ooxml/POIXMLDocumentPart.cs

Lines 419 to 420 in 200c5d3

	PackageRelationship pr = part.GetPackagePart().GetRelationship(id);
	AddRelation(pr, part);

In line 419 the relation is not exists and then it's not checked for null, which cause NRE. Because relation is not exists, we shouldn't call this method at all

[Bykiev]

It seems I was wrong and AddRelation call shouldn't be removed, just added a null check

[cp1319421]

So do I have a new version to try？@Bykiev

[tonyqus]

@cp1319421 NPOI doesn't have nightly build. Please pend the next release. It can be a few month later.

[tonyqus]

@Bykiev I debug into the code and found that the exception occurs in cloning drawing relationship.

It tried to look for rId1. And drawing1.xml.rels looks like this

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
	<Relationship Id="rId3" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image" Target="../media/image1.png"/>
	<Relationship Id="rId2" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart" Target="../charts/chart2.xml"/>
	<Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart" Target="../charts/chart1.xml"/>
</Relationships>

In other words, rId1 is there. For some reason , it's not found. We may have to dig into this.