Currently there are some codepaths where the WSL property is being set as nil, this should be a programmer error and we should DCHECK().

For more context see:
https://codereview.chromium.org/2908623004/diff/20001/ios/clean/chrome/browser/ui/toolbar/toolbar_mediator.mm

Ed, I'm assigning to you on the meantime since you tackled the issue along Rohit before and came up with the disconnect pattern. 

I can help with anything you need, or we can also re-assign.