[BUGFIX] Handle parent is roottree while checking pointerSubFieldName

* Checking if we leveled up till roottree without finding configured pointer field
* Generate complete empty DataStructure for "no DS found" as TYPO3 core do not check
  correctly for existence.

Resolve: #631
Release: 12.1.0

Author: opi99

Classes/Configuration/FlexForm/ParsingModifyEventListener.php

@@ -64,6 +64,14 @@ public function initializeDataStructureIdentifier(BeforeFlexFormDataStructureIde
             while (!$pointerValue) {
                 $uidOfHandle = $row['uid'] ?? 'new_' . $row['t3_origuid'];
                 $handledUids[$uidOfHandle] = 1;
+
+                $parentUid = (int) $row[$parentFieldName];
+                if ($parentUid === 0) {
+                    // We are on TreeRootElement => Leave
+                    $pointerValue = '';
+                    break;
+                }
+
                 $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable($tableName);
                 $queryBuilder->getRestrictions()
                     ->removeAll()
@@ -85,7 +93,7 @@ public function initializeDataStructureIdentifier(BeforeFlexFormDataStructureIde
                     throw new FlexFormInvalidPointerFieldException(
                         'The data structure for field "' . $fieldName . '" in table "' . $tableName . '" has to be looked up'
                         . ' in field "' . $pointerFieldName . '". That field had no valid value, so a lookup in parent record'
-                        . ' with uid "' . $row[$parentFieldName] . '" was done. However, this row does not exist or was deleted.',
+                        . ' with uid "' . $parentUid . '" was done. However, this row does not exist or was deleted.',
                         1463833794
                     );
                 }
@@ -131,7 +139,14 @@ public function setDataStructure(BeforeFlexFormDataStructureParsedEvent $event):
         $identifier = $event->getIdentifier();
         if (($identifier['type'] ?? '') === 'combinedMappingIdentifier') {
             $dataStructure = [
-                'sheets' => [],
+                'sheets' => [
+                    'sDEF' => [
+                        'ROOT' => [
+                            'el' => [
+                            ],
+                        ],
+                    ],
+                ],
             ];
 
             if ($identifier['cobinedIdentifier'] !== '') {