Using NotesDatabase.CreateDocumentCollection on an unopened database is known to cause a crash. This issue was reported to Quality Engineering as SPR# AGUD7LPRDW. For safety, use NotesDatabase.IsOpen to test before calling this method.
Dim coll As NotesDocumentCollection
if db.IsOpen Then
Set coll = db.CreateDocumentCollection
...
Other methods for creating empty collections, described in the Domino Designer wiki, are useful in earlier versions, but NotesDatabase.CreateDocumentCollection is faster.