I would recommend adding the following additional check here to fix these occasional errors.   (textMesh??false)

I get this sometimes when I need to disable my converse bubble between clicks when doing actions say giving characters things and I want to temp hide the UI and then bring it back.  Granted I should be able to track down the correct timing of this but I know adding this additional check just fixes the issue 100% which is safer.

void SetMesh(float timeValue, bool undrawingMesh){ //0 == start mesh, < 0 == end mesh, > 0 == midway mesh
if( textMesh == null || (textMesh??false))

I'll add this additional check in the next update! I'm curious, are you disabling the STM component, or the GameObject that STM is on? I do remember some weirdness from enabling/disabling the component a long time ago, so this might be the solution to that, if so. I want to be able to repro this first!