[BUGFIX] PHP7.2: Check value for NULL before count

Resolves: #82275
Releases: master, 8.7, 7.6
Change-Id: I77dc811456f4bbd759e28c27284c4652ad09dd23
Reviewed-on: https://review.typo3.org/53938


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/backend/Classes/Form/InlineStackProcessor.php

@@ -221,11 +221,11 @@ class InlineStackProcessor
     protected function calculateStructureLevel($level)
     {
         $result = false;
-        $inlineStructureCount = count($this->inlineStructure['stable']);
+        $structureCount = $this->getStructureDepth();
         if ($level < 0) {
-            $level = $inlineStructureCount + $level;
+            $level = $structureCount + $level;
         }
-        if ($level >= 0 && $level < $inlineStructureCount) {
+        if ($level >= 0 && $level < $structureCount) {
             $result = $level;
         }
         return $result;
@@ -259,6 +259,9 @@ class InlineStackProcessor
      */
     public function getStructureDepth()
     {
+        if (!isset($this->inlineStructure['stable']) || !is_array($this->inlineStructure['stable'])) {
+            return 0;
+        }
         return count($this->inlineStructure['stable']);
     }