[BUGFIX] Use page layout specific column labels

Labels of columns in page layouts were only translatable with the generic
non-specific key "flux.grid.columns.$columnName".

This patch sets the page layout's grid parent object to be the page layout's form,
so that the resolved parent ID is that of the form.

A column named "one" within a page layout with id "twocol" will thus get
the label key "flux.twocol.columns.one", just as it was for flux 8.

If the backend layout cannot be found because the page template file does
not exist anymore, we need to return NULL from buildBackendLayout()
to get a proper error message.

Resolves: #1468

Classes/Form/Container/Grid.php

@@ -142,7 +142,7 @@ public function buildExtendedBackendLayoutArray(int $parentRecordUid): array
      * @param int $parentRecordUid
      * @return BackendLayout
      */
-    public function buildBackendLayout(int $parentRecordUid): BackendLayout
+    public function buildBackendLayout(int $parentRecordUid): ?BackendLayout
     {
         $configuration = $this->buildBackendLayoutArray($parentRecordUid);
         $configuration = $this->ensureDottedKeys($configuration);
@@ -153,9 +153,12 @@ public function buildBackendLayout(int $parentRecordUid): BackendLayout
         foreach ($this->flattenSetup($configuration, 'backend_layout.') as $name => $value) {
             $typoScriptString .= $name . ' = ' . $value . LF;
         }
+        if ($root->getName() === null) {
+            return null;
+        }
         return new BackendLayout(
             $this->getRoot()->getName(),
-            LocalizationUtility::translate($label)
+            LocalizationUtility::translate($label, $root->getExtensionName())
                 ? $label
                 : 'LLL:EXT:flux/Resources/Private/Language/locallang.xlf:flux.grid.grids.grid',
             $typoScriptString

Classes/Provider/AbstractProvider.php

@@ -342,11 +342,15 @@ public function getGrid(array $row)
                         $gridColumn->setColSpan($object['colspan'] ?? 1);
                     }
                 }
+                $grid->setParent($form);
                 return $grid;
             }
         }
         $grid = $this->grid ?? $this->extractConfiguration($row, 'grids')['grid'] ?? Grid::create();
         $grid->setExtensionName($grid->getExtensionName() ?: $this->getControllerExtensionKeyFromRecord($row));
+        if ($form) {
+            $grid->setParent($form);
+        }
         return $grid;
     }