Oh, since it's appearing black and pixelated, you're probably using a normal shader with UI text? Try creating a new material for your text mesh, and assigning one of STM's UI shaders to it!
STM and Unity UI text should use the same type of code to work, could you show me the code you're using? I'm not 100% what could be causing the dialogue box to empty and sit blank when you assign text to it vs Unity UI