(A) $game_temp is not initialized until new game or Continue are selected, thus you are trying to use a method on a nil variable.
(B) It wouldn't work straight from the title because common events are updated in $game_map ~ meaning $game_map must exist and be updating.
There might be more that I'm missing since this is just off the top of my head, but the bottom line is that you should initialize all your $game_ variables ~ there might be a few you can ignore, but since you're calling Scene_Map you'll need at least all the $game_ variables involvedin operating that. I wouldn't mix common events in with this type of script though - better to design your own scene.