I'm getting this exception from including an (empty) :sass block:

<script>:31: undefined method `append' for nil:NilClass
(NoMethodError) (of class java.lang.String)

at org.fusesource.scalate.jruby.Sass.filter(Sass.scala:70)
at org.fusesource.scalate.RenderContext$class.filter(RenderContext.scala:275)
at org.fusesource.scalate.DefaultRenderContext.filter(DefaultRenderContext.scala:30)
at org.fusesource.scalate.RenderContext$class.value(RenderContext.scala:234)
at org.fusesource.scalate.DefaultRenderContext.value(DefaultRenderContext.scala:30)
at scalate.templates.$_scalate_$hello$dashscalate_jade$.$_scalate_$render(hello-scalate.jade.scala:35)
at scalate.templates.$_scalate_$hello$dashscalate_jade.render(hello-scalate.jade.scala:62)
at org.fusesource.scalate.RenderContext$$anonfun$render$1$$anonfun$apply$mcV$sp$1.apply$mcV$sp(RenderContext.scala:390)
at org.fusesource.scalate.RenderContext$$anonfun$render$1$$anonfun$apply$mcV$sp$1.apply(RenderContext.scala:390)
at org.fusesource.scalate.RenderContext$$anonfun$render$1$$anonfun$apply$mcV$sp$1.apply(RenderContext.scala:390)
at org.fusesource.scalate.RenderContext$class.withUri(RenderContext.scala:446)
at org.fusesource.scalate.DefaultRenderContext.withUri(DefaultRenderContext.scala:30)
at org.fusesource.scalate.RenderContext$$anonfun$render$1.apply$mcV$sp(RenderContext.scala:389)
at org.fusesource.scalate.RenderContext$$anonfun$render$1.apply(RenderContext.scala:389)
at org.fusesource.scalate.RenderContext$$anonfun$render$1.apply(RenderContext.scala:389)
at org.fusesource.scalate.RenderContext$class.withAttributes(RenderContext.scala:420)
at org.fusesource.scalate.DefaultRenderContext.withAttributes(DefaultRenderContext.scala:30)
at org.fusesource.scalate.RenderContext$class.render(RenderContext.scala:388)
at org.fusesource.scalate.DefaultRenderContext.render(DefaultRenderContext.scala:30)
at org.scalatra.scalate.ScalateSupport$class.renderTemplate(ScalateSupport.scala:44)
...

The source:

 def filter(context: RenderContext, content: String) = {

   val sassPaths = List[String]()
   var result: StringBuffer = new StringBuffer
   jruby.put("@result", result)
   jruby.run(
     "require 'haml-3.0.25/lib/sass'",
     "options = {}",
     "options[:load_paths] = ["+sassPaths.map("'"+_+"'").mkString(",")+"]",
     "options[:cache_location] = '"+engine.workingDirectory+"/sass'",
     "options[:style] = " + (if (engine.isDevelopmentMode)
":expanded" else ":compressed") + "",
     "options[:line_comments] = " + (if (engine.isDevelopmentMode)
"true" else "false") + "",
     "options[:syntax] = :"+syntax,
     "content = <<SCALATE_SASS_EOF\n"+content+"\nSCALATE_SASS_EOF",
     "tree = ::Sass::Engine.new(content, options).to_tree",
     "@result.append(tree.render)") match {
     case Right(result) => result.toString
     case Left((exception, errors)) =>
       val err1 = """(?m)([a-zA-Z_0-9-]+[.]s[ca]ss:\d+:.+)$""".r
       val err2 = """([(]s[ca]ss[)]:\d+:.+)$""".r
       errors match {
         case err1(msg) => throw new TemplateException(msg, exception)
         case err2(msg) => throw new TemplateException(msg, exception)
       }
       throw new TemplateException(errors, exception)
   }
 }

So the @result.append seems to be the culprit.

The problem appears to be due to my using:

renderTemplate("WEB-INF/scalate/templates/hello-scalate.jade")

instead of (note the leading slash):

renderTemplate("/WEB-INF/scalate/templates/hello-scalate.jade")

Not sure how this is manifesting itself in the given exception,
though, especially since on removing the :sass block everything works.