Tile length prediction not showing when there is no assembled convoy

[[C] Ranran]

Currently, when starting to assemble a convoy, the first selection does not correctly display information about how many vehicles to add.

This is a bug due to the depot dialog update. (´・ω・｀)
PR#601 fixes it. Please confirm.

[jamespetts]

Thank you for that and apologies for the delay: now incorporated.