/* Pop the top-most state, making sure it is previousState. */
public void PopState(UiState previousState)
{
if (stateStack.Count == 0)
{
throw new Exception("Cannot call PopState when no states are currently visible");
}
if (stateStack.Last() != previousState)
{
throw new Exception("PopState called but top-most state has unexpected value: " + previousState.GetType());
}
stateStack.RemoveAt(stateStack.Count - 1);
previousState.Hide();
}