title: display problem of glmtree with terminal_panel node_bivplot or node_barplot
body: <p>I use different decision trees applying to the same data but most of them can display like the following figure </p>
  
  <p><img src="https://i.stack.imgur.com/x6RCa.png" alt="see here">.</p>
  
  <p>When I use <code>glmtree</code>, there is an error at terminal nodes</p>
  
  <blockquote>
    <p>Error in <code>[.data.frame</code>(X, , which, drop = FALSE) : undefined columns
    selected</p>
  </blockquote>
  
  <pre><code>columncol&lt;-hcl(c(250,10), 200, 60, 1)
  labelcol&lt;-hcl(265, 150, 100, 0.9)
  indexcol&lt;-hcl(270, 65, 65, 0.9)
  #png(file='c:/glmtree-exmaple.png', width = 1024, height = 512)
  plot(glm.model, drop_terminal = TRUE,
        inner_panel = node_inner(glm.model, fill = c(labelcol, indexcol)),
        terminal_panel=node_bivplot(glm.model, pointcol = "black", boxcol = "black", boxfill = "lightgray", bg = "white", cdplot = FALSE), main="glmtree")
  #dev.off()
  </code></pre>
  
  <p>where glm.model is my result obtained from <code>glmtre</code>e function.</p>
  
  <p>If I delete the <code>terminal_panel</code> part, it works. But the information of terminal nodes is important, I was wondering if there is any solution that can display the terminal nodes in <code>glmtree</code> by colourfull boxplot?</p>