Following this tutorial, I tried to make a .gif file from a series of solutions.

However, when I check .png files, one of every three files is just empty.

I get the following warning from GLVis when I run the script:

MFEM Warning: Screenshot: Reading from the front buffer is unreliable. Resulting screenshots may be incorrect.
But there is no suggestion for a better way to save the figure.

What is a proper way to save figures using glvs scripts?