Why not read the error message and do exactly what it tells you to do? The message is quite helpful: "Call open(obj) before calling writeVideo" means you need to put this before your for-loop:
open(writerObj)
There is also a big clue that you close the object, but never open it.
The videowriter documentation also clearly shows that the video object is opened before being written. The documentation is there for everyone to read.
