This issue is currently [being investigated](https://github.com/monero-project/monero-gui/issues/1291). There is, fortunately, a quick and easy workaround, which is starting GUI v0.12 with `./start-gui.sh`. That is:

1. Browse to the directory `monero-wallet-gui` is located.

2. Open a new terminal.

3. Execute -> `./start-gui.sh`

It should now connect to the daemon (monerod) that is already running in a separate terminal.