Debugging "Object not found" popup

[PatrickAWlson]

Getting this in PWCG missions.  Reported mostly in WWI but also once in WWII.  Because no details are provided, at least that I can see, it's tough to debug.  I used good old UNIX command line to grep out a unique instance of every object in the mission.  I wrote some code to beat that against the ARRAS group file.  Everything in the mission is also in the Arras file.  

 

Any ideas?

 

Mission attached that recreates the problem (requires FC)

try 1917-08-26.zip

[PatrickAWlson]

For anybody interested - the popup is misleading.  It seems to happen if you put a moving vehicle into water.  Stationary items like AA guns in water get no complaints.