require a series on a bar chart in the UI (#8726)

superset/assets/src/explore/controlPanels/DistBar.js

@@ -17,6 +17,7 @@
  * under the License.
  */
 import { t } from '@superset-ui/translation';
+import { nonEmpty } from '../validators';
 
 export default {
   controlPanelSections: [
@@ -55,6 +56,7 @@ export default {
   controlOverrides: {
     groupby: {
       label: t('Series'),
+      validators: [nonEmpty],
     },
     columns: {
       label: t('Breakdowns'),