The reason for the approximately 50 character limit is because there’s many tools that display a single line and will truncate it if it’s more than about that length (though really the point of truncation can vary wildly – plenty of tools will let you go twice that before they cut you off). So if your one line summary is too long, it’ll be cut off and harder to understand your commit at a glance.
You always can elaborate in a second paragraph, at any rate.
How much can you really put in 50 characters?
Fix: NPE in customer download component when users – That’s 50 characters. Should I not mention where I fixed the bug?
Fix: NPE when users downloaded customers without s – I think I can get rid of the actor in some cases.
̀ Fix: NPE when downloading customers without select`. The summary I want to give cannot be truncated any further.
Fix: NPE when downloading customers. This fits, but is so vague as to be pointless as a summary, in my view.
I don’t follow the 50 character rule, but to answer your first question: imo, no, don’t mention where you fixed the bug. This is a commit message that is explicitly tied to the place where you fixed the bug. You can go into more detail in the PR if you need to.