Fix an issue when unmounting with the dropdown open

Author: yuters

src/ReactSelectize.ls

@@ -467,7 +467,7 @@ module.exports = create-class do
                         class-name: \dropdown
                         key: \dropdown
                         ref: \dropdown
-                        on-height-change: (height) ~> find-DOM-node @refs[\dropdown-transition] .style.height = "#{height}px"
+                        on-height-change: (height) ~> find-DOM-node @refs[\dropdown-transition] .style.height = "#{height}px" if @refs[\dropdown-transition]
 
                         # NO RESULT FOUND   
                         if @props.options.length == 0