The pilot reported that during the landing roll of his second uneventful landing, the right main landing gear started to shimmy. He slowed the airplane and proceeded towards the nearest taxiway. Before exiting the runway, the pilot went to retract the flaps when he inadvertently raised the landing gear handle. He quickly re-lowered the landing gear handle; however, the right main landing gear had already started to retract. The airplane veered to the right, exited the runway surface, and came to rest on the grass. The airplane's right wing was substantially damaged. The pilot reported no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.