Add a null check for session tree in getValue (#233)

* add null check

* add no-sandbox

Author: molly-moen

Gruntfile.coffee

@@ -56,6 +56,8 @@ module.exports = (grunt) ->
           viewportSize:
             width: 1000
             height: 1000
+        puppeteer:
+          args: ['--no-sandbox']
       all:
         urls:
           (for x in grunt.file.expand('test/*.html')

src/controller.coffee

@@ -4197,7 +4197,7 @@ Editor::addEmptyLine = (str) ->
     return str + '\n'
 
 Editor::getValue = ->
-  if @session?.currentlyUsingBlocks
+  if @session?.currentlyUsingBlocks and @session?.tree
     return @addEmptyLine @session.tree.stringify({
       preserveEmpty: @session.options.preserveEmpty
     })