I check out a previous revision of a file with magit, that creates a file such as foobar.txt~hash~ in which buffer-file-name is nil. Then something calls file-truename on this and it breaks.
Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
file-truename(nil)
lsp-ui-sideline--run()
apply(lsp-ui-sideline--run nil)
timer-event-handler([t 0 0 200000 nil lsp-ui-sideline--run nil idle 0])