App: fixes #6840: Crash in ObjectIdentifier::resolveProperty

src/App/Document.cpp

@@ -1170,7 +1170,7 @@ void Document::_checkTransaction(DocumentObject* pcDelObj, const Property *What,
                     return;
                 }
             }
-            if(!pcDelObj)
+            if(!pcDelObj)
                 return;
             // When the object is going to be deleted we have to check if it has already been added to
             // the undo transactions
@@ -1647,6 +1647,7 @@ Document::~Document()
     for (auto it = d->objectMap.begin(); it != d->objectMap.end(); ++it) {
         it->second->setStatus(ObjectStatus::Destroy, true);
         delete(it->second);
+        it->second = nullptr;
     }
 
     // Remark: The API of Py::Object has been changed to set whether the wrapper owns the passed