inclusion of beltlayer mod causes usage of Upgrade Planner to crash.
Errror while running event upgrade-planner::on_player_selected_Area (ID 49)
Error while running event beltlayer::on_built_entity(ID 6)
beltlayer/Editor.lua:225:attempt to index local 'entity' (a nil value)
stack traceback: upgrade-planner/control.lua:809: in function 'player_upgrade'
upgrade-planner/control.lua:585: in function 'on_selected_area'
upgrade-planner/control.lua:562: in function <upgrade-planner/control.lua:561>

Note that this error occured on surface world while trying to upgrade vanilla yellow {belts,undergrounds,splitters} and replace with red (vanilla) versions of each. no items from beltlayer were chosen for replacement either in picking/or replace-with sections of Upgrade Planner's utility item.

edit2: crash occurs even if no undergrounds are chosen for replacement (tried a second time just trying to upgrade belts only.

for reference. upgrade planner used is Klonan's release v1.5.3. belt layer v0.1.2
upgrade planner found at:
https://mods.factorio.com/mod/upgrade-planner/discussion/5ba6ca962d7f1a0009460f44

Thanks for the report. I'm working on upgrade-planner compatibility, but running into some bugs in upgrade-planner itself that make this difficult.