Use `String(value)` instead of `value.toString()` as it supports nulls

frontend/src/metabase/lib/query/field.js

@@ -85,5 +85,5 @@ export function getFieldValues(field: ?Field): FieldValues {
 
 export function getHumanReadableValue(value: Value, fieldValues?: FieldValues = []) {
     const fieldValue = _.findWhere(fieldValues, { [0]: value });
-    return fieldValue && fieldValue.length === 2 ? fieldValue[1] : value.toString();
+    return fieldValue && fieldValue.length === 2 ? fieldValue[1] : String(value);
 }

frontend/src/metabase/parameters/components/widgets/CategoryWidget.jsx

@@ -97,7 +97,7 @@ export default class CategoryWidget extends Component {
                             })}
                             onClick={() => { setValue(rawValue); onClose(); }}
                         >
-                            {humanReadableValue || rawValue.toString()}
+                            {humanReadableValue || String(rawValue)}
                         </li>
                      )}
                 </ul>