title: how to use view2.5d:turtle-view in netlogo
body: <p>When I try to use view2.5d:turtle-view in view2.5d extension I always get NullPointerExpectation.</p>
  
  <pre><code>extensions [view2.5d]
  
  turtles-own[
    energy
  ]
  
  to setup
    ca
    create-turtles 50[
    set color red
    setxy random-xcor random-ycor
    set energy random 1000
    ]
    create-turtles 50[
    set energy random 1000
    setxy random-xcor random-ycor
    ]
    view2.5d:turtle-view "test" turtles [the-turtle -&gt; [energy] of the-turtle]
    reset-ticks
  end
  </code></pre>
  
  <p>The error message:</p>
  
  <p>error (NullPointerException)
   while observer running VIEW2.5D:TURTLE-VIEW
    called by procedure SETUP
    called by Button 'setup'</p>
  
  <p>NetLogo is unable to supply you with more details about this error.  Please report the problem 
   at <a href="https://github.com/NetLogo/NetLogo/issues" rel="nofollow noreferrer">https://github.com/NetLogo/NetLogo/issues</a>, or to bugs@ccl.northwestern.edu, and paste the 
  contents of this window into your report.</p>