I should add that the ball bearings were fed into a rubber bag which lowered itself into place under its increasing weight. There was no risk of anything beyond expensive damage if the balls were dropped in.
The bomb was not really live with the balls removed so lowering the weapon onto its cradle without them fitted didn't really add any risk from manhandling or (heaven forbid) being dropped. The true risk was from fire which could puddle the enriched uranium and the balls added some much needed protection. They would also mitigate the risk of an unplanned HE explosion triggering a more sunny event, but clearly any explosion with radioactive shrapnel would not be fun.