Do not iterate over MultiSelection value of Dict key if it is null

Closes #13375

sao/src/view/form.js

@@ -5440,7 +5440,9 @@ function eval_pyson(value){
                 return value.map(function(e) { return JSON.parse(e); });
             },
             set_value: function(value) {
-                value = value.map(function(e) { return JSON.stringify(e); });
+                if (value) {
+                    value = value.map(function(e) { return JSON.stringify(e); });
+                }
                 this.input.val(value);
             }
         });

tryton/tryton/gui/window/view_form/view/form_gtk/dictionary.py

@@ -241,9 +241,10 @@ class DictMultiSelectionEntry(DictEntry):
         selection.handler_block_by_func(self._changed)
         try:
             selection.unselect_all()
-            for v in value:
-                if v in value2path:
-                    selection.select_path(value2path[v])
+            if value:
+                for v in value:
+                    if v in value2path:
+                        selection.select_path(value2path[v])
         finally:
             selection.handler_unblock_by_func(self._changed)