There is no statute that bans keeping the handgun in the car. That said, if you do not have a permit, than I would not leave it in the car; but, I wouldn't leave it unattended in a car anyway. Now, if you choose to, and the gun is stolen, you may face a civil liability if it is used in a crime. We have had people report their handguns stolen after leaving them in the car..... I just want to shake my head at them... but no, its not illegal.
"One of the common failings among honorable people is a failure to appreciate how thoroughly dishonorable some other people are, and how dangerous it is to trust them." Thomas Sowell