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This crash has been happening quite frequently since this summer
(see crbug.com/1128031).

The crash is at line 41:
  assistant_extra->url = web_contents->GetLastCommittedURL();
Given that we just created |assistant_extra| the line above, it must be
that |web_contents| is invalid.

|web_contents| is created at line 94:
  content::WebContents* web_contents =
      browser->tab_strip_model()->GetActiveWebContents();

It is surprising that this can be nullptr because we check at line 52
to ensure the browser is active (so you'd assume it has active web
content).
But irregardless, I added a null check after line 94.

Bug: 1128031
Test: N/A
Change-Id: Iaabdedf5bb133d608db75a16e96907cc18a0d960
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601276
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839458}