Combo Box Validation And Messagebox

I have a list box where I select "Income" or "Expense". In relation to that, In the next column called "Category", I use the offset formula in " Validation". Depending on whether I choose Income or Expense, I will get different dropdown options for "Category". What I want to do is if I choose the first column combo box, and leave the second column combo box empty, or Vice Versa, I want a messagebox popup saying that I have to choose an option from the dropdown. But if both are empty fields, then do nothing. I have attached the file called "Data Validation Test"