Confirmed. Chris, can you take a look?

Known issue, already being investigated by Alan.

Ah, that must be why Alan told me about it :)

WIP fix, needs test: https://chromium-review.googlesource.com/c/chromium/src/+/981839

I think we need to plumb our BrowserViewButtonProvider through the code base more to avoid crashes like these. The existing code has been written with the main UI toolbar in mind and assumes its existence e.g. browser_view->toolbar().

Created issue 826596 for investigating other potential crashes of this nature.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ffa2f3373e77437f51b3923a3d9de9e2ca35135b

commit ffa2f3373e77437f51b3923a3d9de9e2ca35135b
Author: Alan Cutter <alancutter@chromium.org>
Date: Thu Mar 29 06:06:08 2018

Fix crash when uninstalling extension from PWA window

This CL fixes a null pointer crash when uninstalling an extension from
the app menu of a PWA window. The existing code expected the main UI
toolbar to be present.

Bug:  825554 
Change-Id: I89353dd246168393747cca02f02444859b4457c6
Reviewed-on: https://chromium-review.googlesource.com/981839
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546743}
[modify] https://crrev.com/ffa2f3373e77437f51b3923a3d9de9e2ca35135b/chrome/browser/ui/views/extensions/extension_uninstall_dialog_view.cc
[modify] https://crrev.com/ffa2f3373e77437f51b3923a3d9de9e2ca35135b/chrome/browser/ui/views/extensions/extension_uninstall_dialog_view_browsertest.cc
[modify] https://crrev.com/ffa2f3373e77437f51b3923a3d9de9e2ca35135b/testing/buildbot/filters/mash.browser_tests.filter