SDocumentGraphImpl.createTimeline() throws NPE when STextualDS(s) but no tokens exist

[sdruskat]

Environment

Observed in 3.0.6, still valid in develop.

Expected Behaviour

Whenever I have an object of type SDocumentGraph (independent of its state), I can call SDocumentGraph.createTimeline(), which will result in adding an STimeline object to the graph. Afterwards, when I call SDocumentGraph.getTimeline(), a non-null STimeline object is returned.

Actual behavior

When I add n STextualDSs to an SDocumentGraph and call #createTimeline() on it, an NPE is thrown here:

for (STextualDS sTextualDS : getTextualDSs()) {
    sTimeRelList.addAll(sTimeRelTable.get(sTextualDS));
}

I.e., List#addAll() does not support null arguments. At this point, though, sTimeRelTable.get(sTextualDS) returns null as STextualDSs are only added to the Hashtable while iterating over the graph's STextualRelations as returned by #getTextualRelations(). Because there are no STextualRelations in the graph as of yet (n STextualDSs have been added, but no STokens yet!), the STextualDSs in question are never added!

Steps to reproduce

SDocumentGraph graph = SaltFactory.createSDocumentGraph();
graph.createTextualDS("");
graph.createTimeline();