Null is not an object error messsage

Hello,

I am getting null is not an object error code when exporting my short animation.

I have explored previous similar posts and have tried creating a new file and copying layers into it but this hasn't worked.

It's strange as it was working fine until I had to open the file to make a small amend. I made the amend and now it won't export due to error message.

Does anyone have any idea what I could have done wrong? I'm not using scripting and do not know how to script.

Thanks so much!