When you log out of your farmer account, the game will automatically assign a farmer role to another online character so that there are always two farmers online, that's how it was intended.
When you log in again, it's nearly impossible to miss the red or green button on the minimap or the message in chat indicating whether you're a farmer or not.
Regarding your suggestion, it is impossible due to the existence of characters whose primary race is designated as a shaman race.
An idea would be to auto-open the window when you disconnect from a farmer, but that would spam all characters, and maybe some people don't want it to be shown at all, because they're not doing any PvM stuff at the time, so the only way left is to be cautious and adapt to the system.
We'll do our best to improve it over time, if you have any other suggestions that can improve it without negatively impacting other players, please let us know, we'd love to hear them!