Why are questions about PHP notices off-topic for Drupal Answers? Is there a way to make them on-topic?
Questions about PHP notices are of two kinds:
Questions asking what they did wrong when using/setting up a module
Since a PHP notice is a warning about code doing something wrong, there isn't anything the user did wrong. It's the code that needs to be re-written to initialize the variable it uses, check the variables it uses have the expected/correct type, or handle exceptions or any other error code in the appropriate way.
Questions asking how to fix a warning
PHP notices are not specific for Drupal, and fixing them require to just analyze the used code. Eventual workarounds for code used by third-party module are not much helpful, since they are not anymore necessary, once the maintainer of the module fixes the module.
There isn't a way to make those questions on-topic. If the purpose of the question would be to create a patch to fix the module code, and provide it to the current maintainer, the question could be acceptable, as long as it requires Drupal knowledge to answer it, and it is not a plain PHP question every user (even the ones not knowing Drupal) could answer.