Handle string value set from domain inversion in MultiSelection fields

Closes #12181
(grafted from cdca201f1dd7c8a25de757ba93b9ff883aa9bac3)

--HG--
branch : 6.0

sao/src/model.js

@@ -1792,6 +1792,12 @@
             return value;
         },
         set_client: function(record, value, force_change) {
+            if (value === null) {
+                value = [];
+            }
+            if (typeof(value) == 'string') {
+                value = [value];
+            }
             if (value) {
                 value = value.slice().sort();
             }

tryton/tryton/gui/window/view_form/model/field.py

@@ -243,6 +243,10 @@ class MultiSelectionField(Field):
         return value
 
     def set_client(self, record, value, force_change=False):
+        if value is None:
+            value = []
+        if isinstance(value, str):
+            value = [value]
         if value:
             value = sorted(value)
         super().set_client(record, value, force_change=force_change)