This happened while loading a save.

The mod Module Inserter (5.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event ModuleInserter::on_configuration_changed
ModuleInserter/control.lua:487: attempt to index field '_pdata' (a nil value)
stack traceback:
ModuleInserter/control.lua:487: in function 'func'
flib/migration.lua:63: in function 'run'
flib/migration.lua:87: in function 'on_config_changed'
ModuleInserter/control.lua:521: in function <ModuleInserter/control.lua:520>

Thanks for the report, should be fixed in the latest update.
Any chance that save had an older version of Module Inserter in it (4.x) ? In my recent rewrite i forgot to add migrations from older versions.

If it still errors and you don't mind losing configurations from ModuleInserter, just load the save without it, save it and then add ModuleInserter again.

It was 4.0.4 i think.

Throws the next error now - I pasted it below but don't worry about it. Saving and loading as you suggested works fine :)
Thanks for the quick fix and reply btw!

Error while running event ModuleInserter::on_configuration_changed
flib/gui.lua:410: attempt to index field 'flib' (a nil value)
stack traceback:
__flib/gui.lua:410: in function 'update_filters'
flib/gui.lua:346: in function 'build'
ModuleInserter/scripts/gui.lua:554: in function 'create_main_button'
ModuleInserter/control.lua:459: in function 'init_player'
ModuleInserter/control.lua:464: in function 'init_players'
ModuleInserter/control.lua:563: in function 'func'
flib/migration.lua:63: in function 'run'
flib/migration.lua:87: in function 'on_config_changed'
ModuleInserter/control.lua:659: in function <ModuleInserter/control.lua:658>