In the Location Coordinates Page: "Note: if the screen is white after traveling to an area, simply open up the explorer kit in game and go underground. Without moving from your underground position, go back up to the surface. This should fix the problems."
Not only that, but if you don't want to save every time to go underground, the pokécenter and pokémart I believe are always visible. If you go inside one of those and come out, then the map will return to normal. The white screen always happens to me when I'm using the walk-through walls cheat on my game, and doing what I've mentioned fixes the graphics. Just make sure not to hit any of those little striped boxes, because I think the game will freeze if you walk into certain ones.