Hello!
The property mentioned in the error message is actually only used when setting up actions, which usually occurs within battle. Although it does expect there to be a target for the action, It is accessed when preparing and executing an action. So, in theory, starting battle with no enemies is not enough for the game to crash like this. It is more likely the SRPG gear plugin is reusing the Game_Action class for some other purpose.
I could add an exception handling to prevent the game from crashing at least. To do that, however, I'll need a print from the full error in the console. But I can't guarantee they will end up being fully compatible nonetheless, as there may be other changes from SRGP that may not sit well with TAA_EnemyLevels.