Admin Bar: Fail gracefully when adding events to non-existent elements

[obenland]

When testing if addEventListener or attachEvent are available for an object it just checks for it to be truthy (source), which works if it is available, if it's not it creates a type error:

TypeError: Cannot read property 'addEventListener' of null

This can occur in circumstances where jQuery is not available and plugin/themes remove nodes that admin-bar.js expects to be available. To reproduce install & activate ​Logged Out Admin Bar and visit your test site with a theme enabled that does not use jQuery, like ​Responsive Kubrick.

While it's technically not a core bug, let's make sure it fails a little more gracefully when it does encounter these circumstances.

[pento]

Would it be better to test that obj is the type of thing we want, rather than testing the function? It took me a while to understand what the problem was and why 44526.diff​ fixed it.

For example, the addHoverClass() function a little further below addEvent() tests that the t parameter exists before it's used.

[obenland]

I don't think I necessarily have a preference either way.

For argument's sake, in addEvent() we don't care what obj is as long as we're able to attach events to it.

Do you feel like testing for obj first would make the code clearer?

[pento]

Yeah, I'm not really fussed either way. Let's go with 44526.diff​.

[obenland]

In 43517:

Toolbar: Fail gracefully when adding events to non-existent elements

Avoids a type error when obj is not set.

Fixes #44526.

[obenland]

In 43556:

Toolbar: Check if obj is set before using it.

Props pento.
See #44526.