fix(dashboard): Ignore null in error handling (#76543)

Sometimes when mapping the errors, the value being applied to the
updated array is null, but since the typeof null is object we try to
recurse into it and iterate over its keys. This results in an error.

If the value is null, we can just skip it

static/app/views/dashboards/widgetBuilder/utils.tsx

@@ -7,6 +7,7 @@ import {t} from 'sentry/locale';
 import type {SelectValue} from 'sentry/types/core';
 import type {TagCollection} from 'sentry/types/group';
 import type {OrganizationSummary} from 'sentry/types/organization';
+import {defined} from 'sentry/utils';
 import {
   aggregateFunctionOutputType,
   aggregateOutputType,
@@ -98,7 +99,9 @@ export function mapErrors(
       return;
     }
     if (Array.isArray(value) && typeof value[0] === 'object') {
-      update[key] = (value as ValidationError[]).map(item => mapErrors(item, {}));
+      update[key] = (value as ValidationError[])
+        .filter(defined)
+        .map(item => mapErrors(item, {}));
     } else {
       update[key] = mapErrors(value as ValidationError, {});
     }