Seems unlikely to recur but on the off chance this is helpful:
MacOSX 10.11.6 Emacs 27.0.91 crashed upon
1 rapidly flipping between frames
2 killing all but one frame
3 switching input sources in the Apple menu bar
unclear whether it crashed after the second or third event.
In GNU Emacs 27.0.91 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1404.47 Version 10.11.6 (Build 15G22010))
of 2020-06-02 built on LA.local
Windowing system distributor 'Apple', version 10.3.1404
System Description: Mac OS X 10.11.6
Error during redisplay: (clear-minibuffer-message) signaled (void-function timerp) ⟵ [24 of these messages follow Loading *...done messages]
Loading /Applications/Emacs-27.0.91.app/Contents/Resources/lisp/leim/leim-list.el (source)...done
Finding pointers to doc strings...done
Source file ‘/Users/devon/emacs/greek-keys.el’ newer than byte-compiled file; using older file
Loading /Users/devon/quicklisp/slime-helper.el (source)...
ad-handle-definition: ‘apropos’ got redefined
Loading /Users/devon/quicklisp/slime-helper.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading loadup.el (source)...done
Package cl is deprecated
value of $LANG: en_US.UTF-8
Major mode: Lisp Interaction
Minor modes in effect:
>> It's possible that commit f14869cd70e61b1908ec88a5e3d4bf21c7d538a0 has
>> fixed this, but I've not been able to reproduce most of these recent
>> crashes, so I can't say for sure.
> Right. Devon, would it be possible for you to test the development
> version of Emacs and see whether these crashes are indeed gone?
More information was requested, but no response was given within a
month, so I'm closing this bug report. If the problem still exists,
please respond to this email and we'll reopen the bug report.