App: fixes issue #7628: Crash after creating Sketch for Body

The problem is that inside Transaction::addObjectNew() a transaction object is deleted before removing it from the container. When deleting the corresponding transactional object (i.e. a DocumentObject or ViewProvider)
it can happen that it e.g. calls Transaction::addOrRemoveProperty() that now finds the dangling pointer in the container.

The safe way is to first remove the object from the container before deleting it.

src/App/Transactions.cpp

@@ -194,9 +194,12 @@ void Transaction::addObjectNew(TransactionalObject *Obj)
     auto pos = index.find(Obj);
     if (pos != index.end()) {
         if (pos->second->status == TransactionObject::Del) {
-            delete pos->second;
-            delete pos->first;
+            // first remove the item from the container before deleting it
+            auto second = pos->second;
+            auto first = pos->first;
             index.erase(pos);
+            delete second;
+            delete first;
         }
         else {
             pos->second->status = TransactionObject::New;