3399529: Fix fatal.

Closes #3399529

src/Plugin/Field/FieldType/TablefieldItem.php

@@ -161,14 +161,14 @@ class TablefieldItem extends FieldItemBase {
       $values['rebuild'] = $values['tablefield']['rebuild'];
       $values['value'] = $values['tablefield']['table'];
       unset($values['tablefield']);
-      unset($values['rebuild']['rebuild']);
+      unset($values['tablefield']['rebuild']);
     }
     // In case cell_processing is enabled
     // text_format puts values under an extra 'value' key.
     elseif (!empty($values['value']['tablefield'])) {
       $values['rebuild'] = $values['value']['tablefield']['rebuild'];
       $values['value'] = $values['value']['tablefield']['table'];
-      unset($values['rebuild']['rebuild']);
+      unset($values['value']['tablefield']['rebuild']);
     }
     // In case this is being loaded from storage recalculate rows/cols.
     elseif (empty($values['rebuild'])) {
@@ -275,4 +275,4 @@ class TablefieldItem extends FieldItemBase {
     return TRUE;
   }
 
-}
+}
\ No newline at end of file