I have also seen the “null” when I created a new item and linked to channel.
I think you need to wait 1h or 2h until next update. Depends on your refresh setting.
FYI if you already have InfluxDB then it’s fine. I’m using now “In Memory” persistence. This is easy to setup and no need for additional/external tools:
“In Memory” does not support “Restore at Startup”. But to show future values this is not needed in my opinion. I’m happy with it 
Configuration “In Memory”:
Then create a chart in MainUI and make this setting:
