Bug: add_fixed_in_frame_mobjects in ThreeDScene is not working anymore

[kolibril13]

When running this:

from manim import *
class Text3D(ThreeDScene):
    def construct(self):
        axes = ThreeDAxes()
        self.set_camera_orientation(phi=75 * DEGREES,theta=-45*DEGREES)
        text=TextMobject("This is a 3D text")
        self.add_fixed_in_frame_mobjects(text)
        self.add(axes)
        self.add(text)
        self.wait()

I get the following error:
AttributeError: 'tuple' object has no attribute 'sort'
I guess that is related to the z_index implementation, #122 , @PgBiel ?

[PgBiel]

1. May you please provide a more complete log, indicating where the error occurred?
2. (Regarding your question) Does replacing mobjects.sort(...) (camera.py, somewhere around line 418) by mobjects = sorted(mobjects, ...) fix it?

[kolibril13]

Yes, it fixes it!
I will make a pull request.