fitNullModel fails if AnnotatedDataFrame contains a tibble

[smgogarten]

fitNullModel requires manipulation of rownames, which are not supported in tibbles. Add a coercion of tibble to base data.frame here: https://github.com/UW-GAC/GENESIS/blob/master/R/fitNullModel.R#L45

[smgogarten]

Fixed in c9804f2