fix(event-flow-viz): handle null metadata selection (#11885)

* Initialize object with empty array if is multiselect, fix 500 BE error

* modify data coming

Author: maloun96

superset-frontend/src/explore/components/controls/SelectControl.jsx

@@ -102,10 +102,9 @@ export default class SelectControl extends React.PureComponent {
   // Beware: This is acting like an on-click instead of an on-change
   // (firing every time user chooses vs firing only if a new option is chosen).
   onChange(opt) {
-    let optionValue = null;
+    let optionValue = this.props.multi ? [] : null;
     if (opt) {
       if (this.props.multi) {
-        optionValue = [];
         opt.forEach(o => {
           // select all options
           if (o.meta === true) {

superset/viz.py

@@ -2701,7 +2701,7 @@ def query_obj(self) -> QueryObjectDict:
         entity_key = form_data["entity"]
         meta_keys = [
             col
-            for col in form_data["all_columns"]
+            for col in form_data["all_columns"] or []
             if col != event_key and col != entity_key
         ]