Roujin opened the ticket and wrote:

When the train list of a waypoint is emptied, the button for servicing and sending to depot is disabled, but if it was already clicked on, the dropdown stays open (in contrast to other train lists where an open dropdown is immediately closed in this case).
When you then click on "Service" or "Go to depot" (while the list is empty), an assert is triggered.

File: vehicle_gui.cpp
Line: 1040
Expression: this->vehicles.Length() != 0

To reproduce this: Build a waypoint, build a depot, build train in depot, give train order to go to the waypoint. Open the waypoint window. Open the waypoint train list window. (The train should be in the list.) Open the "Service"/"Go to depot" dropdown. Sell the train in its depot. Click on either "Service" or "Go to depot" in the dropdown of the waypoint's (now empty) train list.

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/2249