How to fix error Undefined index: parent in taxonomy_form_term_submit()?
There's a couple of ways you could go about the problem:


Patch Core: If there's no outstanding issue for the problem, alter core, create a patch &amp; bug report, and submit your patch. You can manage your patch in your deployment process until the patch is accepted upstream.
Custom Module: Looking at the comment above the code you posted, it was authored with the intention that someone might need to alter the form ("Contrib modules can then intercept before hook_form_alter to provide scalable alternatives."). You can use a HOOK_form_taxonomy_form_term_alter invocation to alter the submission handler or the form array to mitigate the warning introduced.