Attempt to fix RMessageView crash

[tonisevener]

Phabricator:
https://phabricator.wikimedia.org/T323163

Notes

This is an attempt to fix our RMessageView crashes. I cannot reproduce it, but I suspect the superview reference when setting up the constraints is occasionally nil, because when I replace that parameter with nil I see the same crash and call stack.

I think the better fix would be to have the superview set up all of these constraints, but that's too big of a change at this point in the release. So for now I'm just adding some defensive work to stop the display entirely if the superview is nil to avoid the crash.

Test Steps

1. Regression test these toasts - subscribe/unsubscribe to a talk page topic, go to airplane mode, and confirm messages display fine as before.
2. Force a situation where the new code is run with comments:

      //if (self.superview) {
      //    [self setupLayout];
      //} else {
          [self removeFromSuperview];
          return nil;
      //}

and repeat Step 1. Confirm toasts no longer display, and there are no crashes.

[tonisevener]

This line is an unrelated cleanup thing - I think it was only needed with the way the old constraints were set up, which were modified in #4384. I don't see a need for this call at this point, plus we're calling layoutIfNeeded it again further down in the method after the constraints are set up. Removing it seems to cause no regressions in my testing.

[staykids]

Nice – this bit of defensive programming doesn't seem unreasonable in an attempt to resolve the crash. Haven't yet noticed regressions on iPhone or iPad. Looking at this RMessage library again makes me want to pick up my toast library work again the next time it makes sense to...