Use model attribute to compute model name of CharColumn

Closes #13093
(grafted from 032f64eb30c141103210c1023dd77f96e492c55f)

--HG--
branch : 6.0

sao/src/view/tree.js

@@ -2196,7 +2196,7 @@
             return this.attributes.name;
         },
         get model_name() {
-            return model.name;
+            return this.model.name;
         },
         get_cell: function() {
             var cell = jQuery('<div/>', {