The Pistol star is not a main sequence star, which means it's not modeled properly in-game. Currently, the game's stellar evolution model only works for main sequence stars of about 11 solar masses or less, anything bigger just explodes instantly if "realistic" is on.
To fix this, either turn off "realistic" mode in the star's properties before saving it, or place the star in your simulation when it's paused, and turn off "realistic" mode before you unpause.