Thank you so much! This was very helpful!
Essentaily the only place where I use look_at() is for disapearing turtle's GPUParticles3D to face the player and after the player takes control of the disapearing turtle, the player and the GPUPartiles3D have the same position and thus look_at() fails because it needs a nonzero length to work, which I definetely should have checked first.
But that's more of a consequence of your bug that lets you take control of disapearing turtle.