How to deactivate a dropdown list when changing UI tabs

How can I deactivate my dropdown list when I'm changing between different UI tabs in Unity? The UI tabs are in the same scene.

When I go to a different window in the same scene the dropdown is still unfolded so I want to deactivate it.

I always get the error: "Coroutine couldn't be started because the the game object 'DropdownName' is inactive!" when I try execute the DropdownName.Hide function.