It's not supposed to do it automatically on being added, but it will do it after any default value is changed or text is changed on the mesh - (The default assigned values on a new instance of the script will no nothing - autoLineCount is set to false, lineCount is set to 1) I did it this way because the code called upon OnEnable() and OnDisable() wouldn't hook into STM properly if run on the same frame- so I figured two birds one stone - if the script when created is in a non-automatic state and requires set up, the action of setting it up will finish the enable process once the text mesh is edited. So... it's by design!
There shouldn't be any null errors though, I can give that another look after dinner...
Your "lines per text mesh" value is 31, try changing that, if it's higher than the amount of lines, no additional meshes will be created.