I ran into this issue coming into a couple of different battles, and downloaded a fresh copy of reborn and the allgen mod and am still seeing it. It looks like the specific move that was causing an issue was Light Screen.
[Pokemon AllGen 19.5.34]
Exception: NoMethodError
Message: undefined method `maxpp' for nil:NilClass
PBMove:38:in `maxpp'
PBMove:13:in `totalpp'
PBMove:9:in `initialize'
/Applications/Reborn.app/Contents/Game/patch/Mods/Pokemon.rb:369:in `new'
/Applications/Reborn.app/Contents/Game/patch/Mods/Pokemon.rb:369:in `block in resetMoves'
/Applications/Reborn.app/Contents/Game/patch/Mods/Pokemon.rb:366:in `each'
/Applications/Reborn.app/Contents/Game/patch/Mods/Pokemon.rb:366:in `resetMoves'
/Applications/Reborn.app/Contents/Game/patch/Mods/AllGen Reborn.rb:2027:in `block in pbLoadTrainer'
/Applications/Reborn.app/Contents/Game/patch/Mods/AllGen Reborn.rb:1994:in `each'
/Applications/Reborn.app/Contents/Game/patch/Mods/AllGen Reborn.rb:1994:in `pbLoadTrainer'
/Applications/Reborn.app/Contents/Game/patch/Mods/AllGen Reborn.rb:1187:in `pbTrainerBattle'
(eval):1:in `pbExecuteScript'
Interpreter:266:in `eval'
Interpreter:266:in `pbExecuteScript'
Interpreter:726:in `command_111'
Interpreter:342:in `execute_command'
Interpreter:214:in `block in update'
Interpreter:117:in `loop'
Interpreter:117:in `update'
Scene_Map:106:in `block in update'
Scene_Map:104:in `loop'
Scene_Map:104:in `update'
Scene_Map:70:in `block in main'
Scene_Map:67:in `loop'
Scene_Map:67:in `main'