This is a new issue as of the most recent Transcendence 1.9 alpha. The issue seems to point at something in their OnDockObjAdj code, according to George:

			<OnDockObjAdj>
				(block nil
					(objSetData gSource 'aeZyrranPortAuthority (random 1 20))
					(objSetData gSource 'aeZyrranChecked nil)
				)
			</OnDockObjAdj>
relanat 7 Aug 2019:

Based on the sole use of <OnDockObjAdj> in 1.9a1 source (baStKatsArcologyBase) there may need to be a Nil option in the code now.

This seems to fix it even for multiple overriding dockscreens, eg, illegal items and radioactive ship.

<OnDockObjAdj>
	(block nil
		(objSetData gSource 'aeZyrranPortAuthority (random 1 20))
		(objSetData gSource 'aeZyrranChecked nil)
		Nil
	)
</OnDockObjAdj>

I can't find any documentation on this Event but based on the St Kats arcology code it may be designed to return an object. 1.9a1 seems to have tightened up on non-conforming values so if an object or Nil isn't returned it might be giving the error/causing the crash because of that.
But that is a complete guess!

assumedpseudonym 9 Aug 2019:

 …And apparently a correct one! I’ll have a TBR update up later today to fix that.