title: Replacing NULL values in a data.frame
body: <p>I am quite new to R and I am working on a data frame with several NULL values. So far I am not able to replace those, and I can't wrap my head about a solution so it would be amazing if anybody could help me.</p>
  
  <p>All the variables where the NULL value comes up are classified as factor.</p>
  
  <p>If I use the function is.null(data) the answer is FALSE, which means that the have to replaced to be able to make a decent graph.</p>
  
  <p>Can I use set.seed to replace all the NULL values, or I need to use a different function?</p>