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Currently modules using Nb.Explorer.Folder* need to check for null, perhaps also try Tree.icon, and if that is null too then bundle a default icon image. See e.g. UIUtil in apisupport/project for an example. This is quite clumsy and has performance impact. Suggest ensuring that Nb.Explorer.Folder* have non-null values on all L&F's. This might be the case even today, but it needs to be stated clearly in the documentation. I can help with the implementation and/or write a test for it.
BTW it doesn't seem to be the case for the Ocean L&F.