Variables: Fixes error with: cannot read property length of undefined (#31458) (#31527)

* Variables: Fixes error with: cannot read property length of undefined

* A different fix

(cherry picked from commit e9d2592481)

Co-authored-by: Torkel Ödegaard <torkel@grafana.org>

public/app/features/variables/query/operators.ts

@@ -9,6 +9,7 @@ import { DataSourceApi, FieldType, getFieldDisplayName, isDataFrame, MetricFindV
 import { updateVariableOptions, updateVariableTags } from './reducer';
 import { getTimeSrv, TimeSrv } from '../../dashboard/services/TimeSrv';
 import { getLegacyQueryOptions, getTemplatedRegex } from '../utils';
+import { getProcessedDataFrames } from 'app/features/query/state/runRequest';
 
 export function toMetricFindValues(): OperatorFunction<PanelData, MetricFindValue[]> {
   return (source) =>
@@ -23,6 +24,7 @@ export function toMetricFindValues(): OperatorFunction<PanelData, MetricFindValu
           return frames;
         }
 
+        const processedDataFrames = getProcessedDataFrames(frames);
         const metrics: MetricFindValue[] = [];
 
         let valueIndex = -1;
@@ -30,7 +32,7 @@ export function toMetricFindValues(): OperatorFunction<PanelData, MetricFindValu
         let stringIndex = -1;
         let expandableIndex = -1;
 
-        for (const frame of frames) {
+        for (const frame of processedDataFrames) {
           for (let index = 0; index < frame.fields.length; index++) {
             const field = frame.fields[index];
             const fieldName = getFieldDisplayName(field, frame, frames).toLowerCase();
@@ -181,6 +183,7 @@ export function areMetricFindValues(data: any[]): data is MetricFindValue[] {
   }
 
   const firstValue: any = data[0];
+
   if (isDataFrame(firstValue)) {
     return false;
   }