How to handle conditional dropdown lists

My form has two fields.

The first one is a dropdown list displaying a list of categories, and the second one is a data list that displays a list of users. The data list options depend on the category selected in the dropdown list.

E.g.:

• User selects category A, data list shows options 1-2-3,
• User selects category B, data list shows options 4-5-6.

Here are the questions:

• Should I disable the data list until a category has been selected in the dropdown list?

• What should happen if the user changes the category after selecting an option in the data list?

  • Should the option in the data list be removed from the field?
  • Should the field be in an error state?