I have the same problem. It is the same as the 4.9 theme editor bug, only not fixed in 4.9.1.
Are you suggesting it may be an new incompatibility with older versions of PHP?
Is there a specific REQUIRED version of PHP that MUST be running as of 4.9.1? As soon as I updated from a prior version, I got the error. I know I probably need to update PHP, but what exactly is the absolute oldest version that can be used with 4.9.1?
