title: grid.arrange from gridExtras exiting with "only 'grobs' allowed in 'gList'" after update
body: <p>I just updated R, R Studio, and a bunch of packages including ggplot2 and gridExtras on my Mac. Now gridExtras is failing in basic plotting with the error:</p>
  
  <p>"only 'grobs' allowed in "gList""</p>
  
  <p>Here's some code that should work but does not:</p>
  
  <pre><code>library(ggplot2)
  p1 = qplot(1:10,rnorm(10))
  p2 = qplot(1:10,rnorm(10))
  library(gridExtra)
  grid.arrange(p1, p2, ncol=2, main = "Main title")
  </code></pre>
  
  <p>This dumps out the following error:</p>
  
  <pre><code>Error in gList(list(grobs = list(list(x = 0.5, y = 0.5, width = 1, height = 1,  : 
    only 'grobs' allowed in "gList"
  In addition: Warning message:
  In grob$wrapvp &lt;- vp : Coercing LHS to a list
  </code></pre>
  
  <p>Any help is appreciated!</p>