Fix#2934. Null is passed to StringHelper::strtolower which expects string. Same problem appears to be applicable to HLOOKUP.
I noted the following problem in the code, but will document it here as well. Excel's results are not consistent when a non-numeric string is passed as the third parameter. For example, if cell Z1 contains `xyz`, Excel will return a REF error for function `VLOOKUP(whatever,whatever,Z1)`, but it returns a VALUE error for function `VLOOKUP(whatever,whatever,"xyz")`. I don't think PhpSpreadsheet can match both behaviors. For now, it will return VALUE for both, with similar results for other errors.
While studying the returned errors, I realized there is something that needs to be deprecated. `ExcelError::$errorCodes` is a public static array. This means that a user can change its value, which should not be allowed. It is replaced by a constant. Since the original is public, I think it needs to stay, but with a deprecation notice; users can reference and change it, but it will be unused in the rest of the code. I suppose this might be considered a break in functionality (that should not have been allowed in the first place).
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