front: fix app crashing when power restriction undefined in train sch… (#4107)

…edule

close #4105

Author: SharglutDev

front/src/applications/operationalStudies/components/ManageTrainSchedule/PowerRestrictionSelector.tsx

@@ -139,7 +139,9 @@ export default function PowerRestrictionSelector() {
           options={powerRestrictions}
           onChange={(e: selectorOption) => definePowerRestrictionRange(e.key)}
           selectedValue={
-            (powerRestriction && powerRestriction[0].power_restriction_code) ||
+            (powerRestriction &&
+              powerRestriction[0] &&
+              powerRestriction[0].power_restriction_code) ||
             t('noPowerRestriction')
           }
         />