Upgraded to 1.0.3 and enabled Space Age at the same time. Opened an editor scenario which was created without Space Age. Got this:

The mod YARM - Resource Monitor (1.0.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event YARM::on_player_created (ID 28)
YARM/resmon/ui.lua:109: attempt to index field 'ui' (a nil value)
stack traceback:
YARM/resmon/ui.lua:109: in function 'update_filter_buttons'
YARM/resmon/ui.lua:82: in function 'get_or_create_hud'
YARM/resmon/ui.lua:201: in function 'migrate_player_data'
YARM/resmon.lua:86: in function 'init_player'
YARM/resmon.lua:50: in function <YARM/resmon.lua:48>

All 1.0.x versions crash with the same error.

Sounds like a migration issue in YARM, those are the hardest to test properly so I'm not entirely suprised. Any chance you could send me the save or scenario for an easier repro?

Oh, fancy, I found a different bug while trying to test this:

Error while running event YARM::on_configuration_changed
__YARM__/resmon/ui.lua:206: attempt to index field 'YARM_toggle_surfacesplit' (a nil value)
stack traceback:
    __YARM__/resmon/ui.lua:206: in function 'migrate_player_data'
    __YARM__/resmon.lua:86: in function 'init_player'
    __YARM__/resmon.lua:42: in function <__YARM__/resmon.lua:38>

My saves from 1.1 are so old, the surface split button didn't exist back then... I should still support this somehow.

Well, I fixed my old saves but didn't run into this bug; I feel like this must be related to the scenario instead somehow. I really do want a file to reproduce with now. Nevermind, just converting one of my old saves to a scenario was enough to get this reproducing immediately. Fix probably coming shortly.

Will be fixed in v1.0.4

1.0.4 works fine for me. Thank you for fixing this.