I had a crash with these two mods when using the Hand Mirror in survival. No issues using it in Creative mode since it doesn't factor in a lot of elements. So here is the crash report! Might be something wrong with the rendering with those mods colliding with DS hand mirror.

crash-2020-10-30_15.43.55-client.txt

The root cause lies in the Potion Core mod. They forgot to check whether the current camera entity was an instance of EntityLivingBase before casting.

I suggest posting a thread on their issue tracker and referencing this issue, as they'd have to fix it on their end for a proper solution.