Put putting in a parking lot even with the rear brake, if the turn is too sharp and the brakes are rapidly locked, it can still throw the bike beyond recovery. Straighten the front wheel and use either front or rear (or both) and the chances are you will not go down. Most, especially newer riders that haven't as yet acquired smooth front brake skills at slow speeds, should rely on the rear brake during parking lots, or on slippery surfaces like grass, etc. Even some experienced riders like to use the rear brake more at slow speeds. Keep working on it, keep learning, riding like a pro is in your future.