mbr and mtr images are blank in post_processing_callbacks_and_animation example

[millerj97]

Description of the modifications

running the example post_processing_callbacks_and_animation, the images displayed for p_mtr and p_mbr are blank
examples/00-post_processing/post_processing_callbacks_and_animation.py

note: I changed config.interactive = False as I was experiencing issues on my machine when set to True

Useful links and references

examples/00-post_processing/post_processing_callbacks_and_animation.py