Finding NULL values in R

List columns of a data frame that contain any NULL values:

df_nulls <- sapply(df, function(x) sum(is.na(x)))
df_nulls[df_nulls>0]

Display results in a nice data frame format by adding as.data.frame():

as.data.frame(df_nulls[df_nulls>0]) %>% 
  rename_at(1, ~'n_nulls')