if you take the grip handle off slowly, the rear takedown pin spring and detent pin will also come out (along with the safety spring and detent pin). The takedown pin should easily slide out. Then put the rear takedown pin in its correct position and reassemble the grip handle (with both springs and detents) and you should be good to go.

See the sticky videos for changing grips and trigger groups to understand the takedown pin springs and detents.

Good luck