PVS-Studio warning V595 for Firefox

nsresult
nsFrameSelection::MoveCaret(....)
{
  ...
  mShell->FlushPendingNotifications(Flush_Layout);

  if (!mShell) {
    return NS_OK;
  }
  ...
}

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'mShell' pointer was utilized before it was verified against nullptr. Check lines: 1107, 1109. nsselection.cpp 1107

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/