App: fix possible dangling pointer in Document::removeObject()

src/App/Document.cpp

@@ -4225,6 +4225,10 @@ void Document::removeObject(const char* sName)
         }
     }
 
+    // In case the object gets deleted the pointer must be nullified
+    if (tobedestroyed) {
+        tobedestroyed->pcNameInDocument = nullptr;
+    }
     d->objectMap.erase(pos);
 }