Thanks for informing me.
It looks like I have traced something of this. I did not get an error, but I did find a rather rare situation: if you do the rescue mission and one of the heroines does not escape, and after that you do a mission of another heroine together with that heroine, and after that you do the mission of the heroine that did not escape, she will still be at the mission. I made a fix for that, which I will release today (1.0.3).
That said, as I did not get an error, it might still be helpful if you send me some more detailed information if you have it, in particular: where exactly did the error appear (e.g., the last screen before you saw the error), or, if you have it, a screenshot of the error.