Error: when converting an object to its numerical form; unable to convert null to a number

i realized the program was never running because of the stimulus page and not the introduction page (where the instructionroutine1 plays), i got caught up trying to debug the easier things first when it didnt need to be debugged. however, now i am stuck with trying to figure out what to put as the default images/videos for when the other stimuli plays (when the stimuli is a movie, then the image stimuli does not show, and vise versa. thread to that conversation is here: Randomize Image and Video thread )