[BUGFIX] EXT:form - fix page drag & drop

Fix JavaScript errors on page drag & drop within the form editor

Resolves: #82337
Releases: master, 8.7
Change-Id: I6e67b05a68273f1ad2d6f851ed5dfa4781d7d5bb
Reviewed-on: https://review.typo3.org/53949


Reviewed-by: Daniel Lorenz <daniel.lorenz@extco.de>
Tested-by: Daniel Lorenz <daniel.lorenz@extco.de>
Reviewed-by: Carlos Meyer <cm@davitec.de>
Tested-by: Carlos Meyer <cm@davitec.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>

typo3/sysext/form/Resources/Public/JavaScript/Backend/FormEditor/TreeComponent.js

@@ -317,12 +317,23 @@ define(['jquery',
                 isAllowed: function (placeholder, placeholderParent, currentItem) {
                     var formElementIdentifierPath, formElementTypeDefinition, targetFormElementIdentifierPath, targetFormElementTypeDefinition;
 
+                    if (typeof placeholderParent === 'undefined') {
+                        return true;
+                    }
+
                     formElementIdentifierPath = getTreeNodeIdentifierPathWithinDomElement($(currentItem));
                     targetFormElementIdentifierPath = getTreeNodeIdentifierPathWithinDomElement($(placeholderParent));
 
                     formElementTypeDefinition = getFormElementDefinition(formElementIdentifierPath);
                     targetFormElementTypeDefinition = getFormElementDefinition(targetFormElementIdentifierPath);
 
+                    if (
+                        targetFormElementTypeDefinition['_isTopLevelFormElement']
+                        && !targetFormElementTypeDefinition['_isCompositeFormElement']
+                    ) {
+                        return false;
+                    }
+
                     if (
                         formElementTypeDefinition['_isGridContainerFormElement']
                         && (