I don't think the old version of the shader had the exact same code as the new one, so I have no idea where you'd be inserting that line. (This line is meant to only be active for UI text specifically, by the way!) Please use the new version of STM I sent you, it should be viable as a release candidate!
When updating shaders/cginc files, you may have to right click on the shader or folder containing the shader and select "Reimport" to update it without needing to restart Unity.
If that error persists, go in to the Text Data Inspector, and click the button at the bottom that says "Refresh Database" and see if that fixes it. Line 6018 doesnt line up with my working build anymore so I'm not sure which line is truly causing that error, meanwhile line 108 in STMQuadData tells me that the texture of the quad is null... Which would be caused by the blank quad I told you to make. Oops... I suppose I meant to use a blank texture in this regard, not actually leave the field null. All the more reason for me to figure out a more concrete answer to altering the size of spaces...
I can update the build and send it again if you like!