error: Only one toplevel sub-topic is allowed to define the root of the topic tree
 --> ui/double_toplevel.rs:4:10
  |
4 |     bus::Toplevel1 => {
  |          ^^^^^^^^^

error: Only one toplevel sub-topic is allowed to define the root of the topic tree
 --> ui/double_toplevel.rs:7:10
  |
7 |     bus::Toplevel2 => {
  |          ^^^^^^^^^