Any way to reproduce this bug? Does it happen randomly?

It happens randomly sadly when I close Bluetooth chooser after clicking "Scan" button at https://jyasskin.github.io/sandbox/notfound.html

Here's another crash: 
See http://crash/cfdb0d2e00000000

Woohoo! I'm able to reproduce it!

- Open a background YouTube tab to https://www.youtube.com/watch?v=iNJdPyoqt8U
- Make sure 1080p video doesn't start yet by opening quickly a new tab to https://jyasskin.github.io/sandbox/notfound.html
- Switch to the YT tab and switch back quickly to the bluetooth tab to trigger video
- Click "Scan" button
- Click "Cancel" button in the bluetooth chooser
- C R A S H !

Note that I can't reproduce it with https://googlechrome.github.io/samples/web-bluetooth/battery-level.html

Looking at the source trace in crash/, it looks like it crashes because bubble_reference_ is already null. See https://chromium.googlesource.com/chromium/src/+/34f9699d9be6dd32931c9e694f61b3022346f4e5/chrome/browser/ui/views/website_settings/chooser_bubble_ui_view.cc#138

Thanks François! I can reproduce this bug. I'll submit a CL to fix it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/76f6b1648624d9fe6ce48b7d5996e3c2d41ec710

commit 76f6b1648624d9fe6ce48b7d5996e3c2d41ec710
Author: juncai <juncai@chromium.org>
Date: Tue Sep 06 20:23:45 2016

Check if BubbleReference weak pointer is valid before using it in the chooser

Since BubbleReference is a base::WeakPtr<BubbleController>, before
using it, needs to check if it is valid. This CL added code to do it.

BUG= 642748 

Review-Url: https://codereview.chromium.org/2309563002
Cr-Commit-Position: refs/heads/master@{#416698}

[modify] https://crrev.com/76f6b1648624d9fe6ce48b7d5996e3c2d41ec710/chrome/browser/ui/cocoa/website_settings/chooser_bubble_ui_cocoa.mm
[modify] https://crrev.com/76f6b1648624d9fe6ce48b7d5996e3c2d41ec710/chrome/browser/ui/views/website_settings/chooser_bubble_ui_view.cc