Thankfully, there is a workaround for this. The good thing, it is very simple and does not need any advanced Linux expertise. Below are the steps:
- Open the System Preferences.
- Open the Display setting that is located at the Hardware section.
- Choose the unknown monitor and DISABLE it.
- Click on the Apply button and choose to keep the configuration when asked.
That's all. Hopefully it's helpful and feel free to add if there are other methods to solve this annoying problem.