You didn't name your instance... alway's name your instances whether your gonna use them or not, makes things easier in the long run.
Naming it the same as the var shouldn't have caused issues.
It's good practice to name your instances with extensions so that you always know what kind of object your dealing with title_txt , title_mc, title_gr etc.
For future reference you can also add text to a text field using the .text method. Good for appending text to a field
_root.title_txt.text = "hello";
_root.title_txt.text += " how are you doing?";