I recently copied the files over from one of my old MV projects into MZ and I get this popup whenever I try and run the game. I've copied all the game's data into MZ; according to the plugin manager, I'm running no plugins.
something is undefined - and that is not the fontfilename itself, but the data object that is the parent of that property.
so for better help we will need the full console output of that error (press F8 when it appears).
But as a first guess I suggest you check the instructions of what to copy and what not to copy and how to update your game when transferring from MV to MZ, because there is a high chance that you made a mistake with those instructions.
in that case you need to go to the browser console manually.
go to the project folder and start the index.html with a browser of your choice.
play until the error message appears
while the error message is displayed, go to the menu of the browser, go to the developer tools there and then select the console (menu structure depends on which browser you're using, but all have the console somewhere on the developer tools)