The issue is probably unrelated to the theme.
You can debug the issue, for instance;
1. Try switching to WordPress standard theme like 2017, consult the host if error occurs.
2. The next thing you can do is deactivate every plugins and reactivating them one by one to check if error exists or not.
The notice error you are getting however doesnot affect anything .You can ignore it for now. We shall fix it in our further updates.