weyl, symmetrize and antisymmetrize functions provide error

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So I am running the Simon-Mars tensor and Kerr spacetime example ipynb without changing anything, and I found that the weyl, symmetrize and antisymmetrize functions are not able to run. The error is the following with a lot of text above it providing the path it took to conclude that it is the specified error.

TypeError: expected str, NoneType found

I am guessing something changed from version 7.5.1 which I see in the pdf of the example where these functions worked well and they are not running now, or I am doing something wrong even though all I do is Kernel > Restart and Run All. Any help would be greatly appreciated.