Fix a null reference exception in parseSeparationColumn

[Mailaender]

java.lang.NullPointerException: Cannot invoke "org.eclipse.chemclipse.model.core.IChromatogram.isParseSeparationColumnEnabled()" because "chromatogramReference" is null
	at org.eclipse.chemclipse.model.support.ChromatogramColumnSupport.parseSeparationColumn(ChromatogramColumnSupport.java:53)
	at org.eclipse.chemclipse.model.support.ChromatogramColumnSupport.parseSeparationColumn(ChromatogramColumnSupport.java:34)
	at org.eclipse.chemclipse.converter.chromatogram.AbstractChromatogramConverter.postProcessChromatogram(AbstractChromatogramConverter.java:269)
	at org.eclipse.chemclipse.converter.chromatogram.AbstractChromatogramConverter.convert(AbstractChromatogramConverter.java:153)
	at org.eclipse.chemclipse.ux.extension.xxd.ui.internal.runnables.ChromatogramImportRunnable.run(ChromatogramImportRunnable.java:103)
	at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:123)

[eselmeister]

That's strange. A chromatogram reference shouldn't be null at all. Normally, it should be checked when adding a reference:

if(reference != null) {
	chromatogram.addReferencedChromatogram(reference);
}