getDefaultCategories() has replaced getDefaultCategory(), but getDefaultCategory() is now ignored, which breaks compatibility with old code defined using the old interface.
Relevant commit: https://github.com/xwiki/xwiki-rendering/commit/40e5d9534e9c5af62ef27f7d2bc8ca9ca3539e8f#diff-5cd2dac768c3cedc5427daa48c5af68488aeb5d81e2a2ecadb849197010fd4ce
