The mod Underground Transport (0.1.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UndergroundTransport::ut-left-click (ID 277)
UndergroundTransport/scripts/gui.lua:73: attempt to index field 'port' (a nil value)
stack traceback:
UndergroundTransport/scripts/gui.lua:73: in function 'openOutputPortGui'
UndergroundTransport/control.lua:131: in function <UndergroundTransport/control.lua:121>

I have two output ports that should push out asteroids. When I clicked on one (on my ship), it crashes my game with this message.

I'm unsure if this is related, but I'm 90% certain it is, I tried to launch another rocket into the same orbit and got this error message:

The mod Underground Transport (0.1.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UndergroundTransport::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__UndergroundTransport/scripts/network.lua:72: in function 'tick'
UndergroundTransport/control.lua:11: in function <UndergroundTransport/control.lua:10>

Basically my save is currently bricked due to those two underground transport tunnels. I'll try uninstalling/reinstalling the mod to see if that helps.

Thanks for the reports! Taking a look now.

For the first issue, I've tried loading metallic asteroid chunks into an input port and outputting them from two output ports, and clicking on any seems to be working for me. What other mods are you using? Maybe there's a conflict or something more subtle.

For the second issue, the issue happened when deploying a new space platform? or was it a cargo rocket delivering cargo to an existing platform?

I think it was the fact that I can stack asteroids from a 4 stack asteroids mod I have? Maybe?

And I got this error when deploying a new space platform, not delivering cargo:

The mod Underground Transport (0.1.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UndergroundTransport::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__UndergroundTransport/scripts/network.lua:72: in function 'tick'
UndergroundTransport/control.lua:11: in function <UndergroundTransport/control.lua:10>

Ah ok - what asteroid stacking mod are you using? I'll check it out.

Thanks for the clarification on the 2nd issue - I'll test that scenario and figure out what's going wrong 👍

https://mods.factorio.com/mod/asteroids-stack-to-four

For the 2nd issue, it only occurs when I try to launch/start a platform in the same orbit of another platform that has these underground tunnels installed.

My save is broken until this has been fixed, unfortunately.

Oh no! I'll look into this today. Feel free to disable this mod or the asteroid stacking mod in the mean time.

By the "same orbit" do you mean launching a new platform starter from the same planet where you have another platform with underground ports? I've tried it a few times but am not able to reproduce the issue. Could you share your save file?