Need a figure() call before the profile uncertainty plot in export

Here's the figure produced by `$ refl1d doc/examples/ex1/nifilm-fit.py --fit=dream -b --export=T1` in the file `T5/nifilm-fit-err.png`:

<img width="579" height="454" alt="Image" src="https://github.com/user-attachments/assets/405adac8-cbdc-43af-a771-ae3977681573" />